Rollup merge of #102353 - bjorn3:allow_rustix_use_libc, r=Mark-Simulacrum

Allow passing rustix_use_libc cfg using RUSTFLAGS

Before this would error with

```
error: unexpected `rustix_use_libc` as condition name
  |
  = note: `-D unexpected-cfgs` implied by `-D warnings`
  = help: was set with `--cfg` but isn't in the `--check-cfg` expected names
```

I'm setting rustix_use_libc when testing bootstrapping rustc with cg_clif as I'm disabling inline asm here.

diff --cc src/bootstrap/lib.rs
Simple merge