Auto merge of #87402 - nagisa:nagisa/request-feature-requests-for-features, r=estebank

Direct users towards using Rust target feature names in CLI

This PR consists of a couple of changes on how we handle target features.

In particular there is a bug-fix wherein we avoid passing through features that aren't prefixed by `+` or `-` to LLVM. These appear to be causing LLVM to assert, which is pretty poor a behaviour (and also makes it pretty clear we expect feature names to be prefixed).

The other commit, I anticipate to be somewhat more controversial is outputting a warning when users specify a LLVM-specific, or otherwise unknown, feature name on the CLI. In those situations we request users to either replace it with a known Rust feature name (e.g. `bmi` -> `bmi1`) or file a feature request. I've a couple motivations for this: first of all, if users are specifying these features on the command line, I'm pretty confident there is also a need for these features to be usable via `#[cfg(target_feature)]` machinery.  And second, we're growing a fair number of backends recently and having ability to provide some sort of unified-ish interface in this place seems pretty useful to me.

Sponsored by: standard.ai

src/lib.rs

@@ -132,7 +132,7 @@ impl ExtraBackendMethods for GccCodegenBackend {
         base::compile_codegen_unit(tcx, cgu_name)
     }
 
-    fn target_machine_factory(&self, _sess: &Session, _opt_level: OptLevel) -> TargetMachineFactoryFn<Self> {
+    fn target_machine_factory(&self, _sess: &Session, _opt_level: OptLevel, _features: &[String]) -> TargetMachineFactoryFn<Self> {
         // TODO(antoyo): set opt level.
         Arc::new(|_| {
             Ok(())