feature gate FFI imports of LLVM intrinsics

[huonw]

E.g. one can import an intrinsic via

extern {
    #[link_name = "llvm.sqrt.f32"]
    fn sqrt(x: f32) -> f32;
}

AFAIK, LLVM doesn't have guarantees about the stability of intrinsics, so a library usable with Rust 1.0 that does the above may become unusable with Rust 1.x if LLVM is upgraded. Being chaine to a specific LLVM version would be really bad, so we should feature gate link_names that start with llvm..

(Tagging as E-mentor, since I'm happy to help. The relevant code is in syntax::feature_gate.)

[nagisa]

I’ll take this.