bootstrap cannot check library/std for windows from macOS (and possibly others)

[thomcc]

I often write patches for std::sys::windows while not using windows. It would be nice if ./x.py check --target x86_64-pc-windows-msvc library/std worked, but instead it does not -- I get an error like https://gist.github.com/thomcc/7c889b3a1306922f7eda28f45f1d304e (this is from a build where aarch64-apple-darwin is the host).

As a result, to check changes to std::sys::windows while not on windows, I'm forced to a full stage1 build, and then in some other workspace run cargo +stage1 check -Zbuild-std --target x86_64-pc-windows-msvc (where +stage1 is a linked toolchain). This works, but is tedious/time-consuming.

CC @jyn514 who suggested I file this issue, and suggested that it "looks like we're handling the linker arguments wrong when cross compiling".

[jyn514]

@thomcc how did you get that far in the build? When I try it I get

; ./x.py check --target x86_64-pc-windows-msvc library/std
thread 'main' panicked at '
cmake does not support Visual Studio generators.

This is likely due to it being an msys/cygwin build of cmake,
rather than the required windows version, built using MinGW
or Visual Studio.

If you are building under msys2 try installing the mingw-w64-x86_64-cmake
package instead of cmake:

$ pacman -R cmake && pacman -S mingw-w64-x86_64-cmake
', sanity.rs:221:17

[jyn514]

download-ci-llvm worked`

[jyn514]

To reproduce this I also needed to have the llvm submodule checked out so

rust/src/bootstrap/compile.rs

Line 319 in ed37111

cargo.env("RUST_COMPILER_RT_ROOT", &compiler_builtins_root);

would trigger.