Add `-Zfixed-x18` This PR is a follow-up to #124323 that proposes a different implementation. Please read the description of that PR for motivation. See the equivalent flag in [the clang docs](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-ffixed-x18). MCP: https://github.com/rust-lang/compiler-team/issues/748 Fixes https://github.com/rust-lang/rust/issues/121970 r? rust-lang/compiler