Make all linker warnings errors in CI

[tgross35]

Give us a better chance of detecting any kind of link issues or user-visible warnings in advance.

Inspired by #665 (comment)

[tgross35]

Failure notes:

• Apple and thumb (via rust-lld) do not seem to support the flag
• powerpc- has errors from bss-plt forced due to ....
• wasm gets a lot of is neither Wasm object file nor LLVM bitcode warnings. These seem to be all the object files from the C code we build, e.g. absvdi2. I am not sure why we build the C versions in the first place, since the readme says they are never called by LLVM. There is some context to why fatal-warnings isn't passed on wasm, not directly related Remove --fatal-warnings on wasm targets rust#120278.
• windows-gnu has corrupt .drectve warnings. Looks like this is a known issue GNU linker warns “corrupt .drectve” on staticlib binary generated with Rust 1.70 on Windows rust#112368

[ChrisDenton]

For MSVC it's /WX

[tgross35]

Does it ignore unrecognized link args? fatal-warnings is still getting passed and it doesn't seem to complain https://github.com/rust-lang/compiler-builtins/actions/runs/10343839608/job/28628523945?pr=667#step:11:23

[ChrisDenton]

I mean... /Wx isn't being passed yet?

> rustc hello.rs -C link-args='/WX --fatal-warnings'

error: linking with `link.exe` failed: exit code: 4044
[...]
  = note: LINK : warning LNK4044: unrecognized option '/-fatal-warnings'; ignored
          LINK : error LNK1218: warning treated as error; no output file generated

[tgross35]

Oh, unrecognized linker args are warnings by default rather than errors? I was just expecting it to complain like the other targets that don't support -Wl,--fatal-warnings do.