Pass tune-cpu to LLVM

I think this is how it should work...

compiler/rustc_interface/src/tests.rs

@@ -583,6 +583,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(symbol_mangling_version, SymbolManglingVersion::V0);
     tracked!(teach, true);
     tracked!(thinlto, Some(true));
+    tracked!(tune_cpu, Some(String::from("abc")));
     tracked!(tls_model, Some(TlsModel::GeneralDynamic));
     tracked!(treat_err_as_bug, Some(1));
     tracked!(unleash_the_miri_inside_of_you, true);