std: Set has_reliable_f16 to false for MIPS targets in build.rs

[martn3]

This PR makes std tests link for MIPS again (they broke with #126608) by avoiding the following link errors. Step-by-step instructions on how to reproduce these errors in docker can be found below.

std.9e27ea-cgu.12:(.text._ZN3std3num8test_num17edc3E+0x38): undefined reference to `__gnu_f2h_ieee'
std.9e27ea-cgu.12:(.text._ZN3std3num8test_num17hdc3E+0x38): undefined reference to `__gnu_h2f_ieee'

This PR just adds one line of config in existing f16 infrastructure. It also disables four doctests that fails with the same link errors.

Step-by-step to reproduce linking error

1. Prepare:

docker run -it ubuntu:24.10

apt update && apt install -y \
    libc6-mips-cross \
    libc6-mipsel-cross \
    libc6-mips64-cross \
    libc6-mips64el-cross \
    gcc-mips-linux-gnu \
    gcc-mipsel-linux-gnu \
    gcc-mips64-linux-gnuabi64 \
    gcc-mips64el-linux-gnuabi64 \
    git curl python3 build-essential

git clone --depth 1 https://github.com/rust-lang/rust.git
cd rust

2. Try to link std tests for any of these 4 MIPS targets by running any one of these commands:

CC_mips_unknown_linux_gnu=mips-linux-gnu-gcc \
CARGO_TARGET_MIPS_UNKNOWN_LINUX_GNU_LINKER=mips-linux-gnu-gcc \
./x test library/std --target mips-unknown-linux-gnu

CC_mipsel_unknown_linux_gnu=mipsel-linux-gnu-gcc \
CARGO_TARGET_MIPSEL_UNKNOWN_LINUX_GNU_LINKER=mipsel-linux-gnu-gcc \
./x test library/std --target mipsel-unknown-linux-gnu

CC_mips64_unknown_linux_gnuabi64=mips64-linux-gnuabi64-gcc \
CARGO_TARGET_MIPS64_UNKNOWN_LINUX_GNUABI64_LINKER=mips64-linux-gnuabi64-gcc \
./x test library/std --target mips64-unknown-linux-gnuabi64

CC_mips64el_unknown_linux_gnuabi64=mips64el-linux-gnuabi64-gcc \
CARGO_TARGET_MIPS64EL_UNKNOWN_LINUX_GNUABI64_LINKER=mips64el-linux-gnuabi64-gcc \
./x test library/std --target mips64el-unknown-linux-gnuabi64

Expected

No link error. After this PR there are no link errors.

Actual

error: linking with `mips-linux-gnu-gcc` failed: exit status: 1
  |
  = note: LC_ALL="C" PATH="/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/bin:/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/bin:/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin" VSLANG="1033" "mips-linux-gnu-gcc" "/tmp/rustcEtKsay/symbols.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.00.rcgu.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.01.rcgu.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.02.rcgu.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.03.rcgu.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.04.rcgu.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.05.rcgu.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.06.rcgu.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.07.rcgu.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.08.rcgu.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.09.rcgu.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.10.rcgu.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.11.rcgu.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.12.rcgu.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.13.rcgu.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.14.rcgu.o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.15.rcgu.o" "-Wl,--as-needed" "-L" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps" "-L" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/release/deps" "-L" "/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/mips-unknown-linux-gnu/lib" "-Wl,-Bstatic" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/librand_xorshift-deb32232a867c543.rlib" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/librand-5a391600dce9d98f.rlib" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/librand_core-a11cfba3d86c5298.rlib" "/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/mips-unknown-linux-gnu/lib/libtest-65b05caf5a9b99a4.rlib" "/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/mips-unknown-linux-gnu/lib/libgetopts-ba692b2f798aef60.rlib" "/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/mips-unknown-linux-gnu/lib/libunicode_width-20ec8b475126cb0b.rlib" "/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/mips-unknown-linux-gnu/lib/librustc_std_workspace_std-c17f739fee51cc86.rlib" "-L" "/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/mips-unknown-linux-gnu/lib" "-Wl,-Bdynamic" "-lstd-124ee57a4c00deda" "-Wl,-Bstatic" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/libcompiler_builtins-bd55a137b89bc81f.rlib" "-Wl,-Bdynamic" "-lgcc_s" "-lutil" "-lrt" "-lpthread" "-lm" "-ldl" "-lc" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/mips-unknown-linux-gnu/lib" "-o" "/rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63" "-Wl,--gc-sections" "-pie" "-Wl,-z,relro,-z,now" "-Wl,-O1" "-nodefaultlibs" "-Wl,-z,origin" "-Wl,-rpath,$ORIGIN/../lib"
  = note: /usr/lib/gcc-cross/mips-linux-gnu/12/../../../../mips-linux-gnu/bin/ld: /rust/build/x86_64-unknown-linux-gnu/stage1-std/mips-unknown-linux-gnu/release/deps/std-1cffa50fa8c43b63.std.9ee227e919a554fa-cgu.12.rcgu.o: in function `std::num::test_num':
          std.9ee227e919a554fa-cgu.12:(.text._ZN3std3num8test_num17haed2ea710c1afdc3E+0x38): undefined reference to `__gnu_f2h_ieee'
          /usr/lib/gcc-cross/mips-linux-gnu/12/../../../../mips-linux-gnu/bin/ld: std.9ee227e919a554fa-cgu.12:(.text._ZN3std3num8test_num17haed2ea710c1afdc3E+0x3c): undefined reference to `__gnu_f2h_ieee'
          /usr/lib/gcc-cross/mips-linux-gnu/12/../../../../mips-linux-gnu/bin/ld: std.9ee227e919a554fa-cgu.12:(.text._ZN3std3num8test_num17haed2ea710c1afdc3E+0x44): undefined reference to `__gnu_h2f_ieee'
          ...
          collect2: error: ld returned 1 exit status

error: could not compile `std` (lib test) due to 1 previous error