Consider deprecating LLVM-specific feature names for -Ctarget-feature flag

[nagisa]

As of #84991 we support the same values for -Ctarget-feature as we do for the #[target_feature] attribute. However there is a fallback that passes through any unknown names, which effectively means that LLVM names are also accepted by the flag.

I think it makes sense for us to look into deprecating LLVM-specific names, and eventually remove the fallback (or destabilize the fallback behaviour).