Auto merge of #76830 - Artoria2e5:tune, r=nagisa

Pass tune-cpu to LLVM

I think this is how it should work...

See https://internals.rust-lang.org/t/expose-tune-cpu-from-llvm/13088 for the background. Or the documentation diff.

compiler/rustc_codegen_ssa/src/traits/backend.rs

@@ -124,4 +124,5 @@ pub trait ExtraBackendMethods: CodegenBackend + WriteBackendMethods + Sized + Se
         opt_level: config::OptLevel,
     ) -> Arc<dyn Fn() -> Result<Self::TargetMachine, String> + Send + Sync>;
     fn target_cpu<'b>(&self, sess: &'b Session) -> &'b str;
+    fn tune_cpu<'b>(&self, sess: &'b Session) -> Option<&'b str>;
 }