LoongArch target features not supported by LLVM 19 are not handled properly #140455
Description
Since #138056 landed, every use of rustc built with LLVM 19 with a loongarch target shows a bunch of warnings about these target features not being supported -- no matter whether the target features are being used or not. From looking at the logic, I think this will affect stable builds as well.
This is not how things should work. As @taiki-e points out, the proper approach for this is to filter out these target features in to_llvm_features.
@heiher can you take care of this? Please also add a comment in target_feature.rs so that there's a higher chance someone can figure this out by themselves in the future.