Create personality stub function for no_std panic=abort crates

[Noratrieb]

This is a very WIP state. It has no tests, no platform gates, and probably many bugs.
I'm opening this PR mostly as a proof of concept.

Every crate that may unwind (being compiled with panic=unwind) needs a reference to a personality function in its object code.

The personality function is defined in std. For crates downstream of std (with std in the cstore), they will resolve to the personality lang item of std and reference that function from there (using its symbol_name rust_eh_personality).

For #![no_std] crates, they will also reference the rust_eh_personality symbol (directly, not via weak lang item machinery).

But when std is never linked into a crate graph containing panic=unwind crates (which happens on all panic=unwind default targets because of core), the symbol isn't present and we get a linker error.

This PR solves this problem by injecting a stub for rust_eh_personality into the final link of binaries where the previous conditions were fulfilled. This is implemented in a way that's very similar to the allocator shim.

Because we don't want to insta-stabilize this functionality, the stub doesn't just abort, it will first do a volatile read of the __rust_personality_stub_is_unstable symbol (once again similar to the allocator) to ensure that this functionality cannot be relied on without providing this symbol.

Example code that now works on x86_64-unknown-linux-gnu:

fn panic_handler(_: &core::panic::PanicInfo<'_>) -> ! {
    loop {}
}

unsafe extern "C" {
    fn write(fd: i32, buf: *const u8, count: usize);
}

static __rust_personality_stub_is_unstable: u8 = 0;

fn main() -> i32 {
    // bring in some code that requires the personality function
    [1].sort_unstable();

    let msg = b"meow\n";
    unsafe {
        write(1, msg.as_ptr(), msg.len());
    }
    0
}

When compiled with -Cpanic=abort, this would previously result in a linker error because the rust_eh_personality symbol was not found. Now, it compiles and runs successfully.

r? @ghost

random notes:

• Björn told me I need to export __rust_personality_stub_is_unstable from rust dylibs

[rust-log-analyzer]

The job x86_64-gnu-llvm-18 failed! Check out the build log: (web) (plain)

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#20 exporting to docker image format
#20 sending tarball 20.6s done
#20 DONE 27.0s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-18]
debug: `DISABLE_CI_RUSTC_IF_INCOMPATIBLE` configured.
---
sccache: Listening on address 127.0.0.1:4226
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-18', '--enable-llvm-link-shared', '--set', 'rust.randomize-layout=true', '--set', 'rust.thin-lto-import-instr-limit=10', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'rust.lld=false', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-18/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.randomize-layout := True
configure: rust.thin-lto-import-instr-limit := 10
---
   Compiling cranelift-native v0.116.1
   Compiling cranelift-frontend v0.116.1
   Compiling cranelift-object v0.116.1
   Compiling rustc_codegen_cranelift v0.1.0 (/checkout/compiler/rustc_codegen_cranelift)
error[E0063]: missing field `personality_stub_module` in initializer of `CodegenResults`
   --> compiler/rustc_codegen_cranelift/src/driver/aot.rs:129:31
129 |         let codegen_results = CodegenResults {
    |                               ^^^^^^^^^^^^^^ missing `personality_stub_module`

For more information about this error, try `rustc --explain E0063`.

[bors]

☔ The latest upstream changes (presumably #127173) made this pull request unmergeable. Please resolve the merge conflicts.