Auto merge of #101833 - jyn514:cross-compile-compiler-builtins, r=Mar…

…k-Simulacrum

Make the `c` feature for `compiler-builtins` an explicit opt-in

Its build script doesn't support cross-compilation. I tried fixing it, but the cc crate itself doesn't appear to support cross-compiling to windows either unless you use the -gnu toolchain:
```
  error occurred: Failed to find tool. Is `lib.exe` installed?
```

Fixes rust-lang/rust#101172.