Auto merge of #98843 - Urgau:check-cfg-stage0, r=Mark-Simulacrum

Enable check-cfg in stage0

Now that the bootstrap cargo supports `rustc-check-cfg` we can now enable it with `-Zcheck-cfg=output` and use it in `rustc_llvm` to unblock `--check-cfg` support in stage0.

r? `@Mark-Simulacrum`

Author: bors