Pre-RFC: is_x86_feature_detected! in core

Summary Make the is_x86_feature_detected! macro available in core on x86 and x86_64 targets. Motivation It's common for crates that only perform computation to sooner or later to get a request to work as no_std crates, so it should be easy for compute-only crates to be no_std crates. Also, (especially when SIMD is used) some crates that perform computation benefit from function multiversioning on x86 and x86_64. Multiversioning builds on the is_x86_feature_detected! macro. is_x86_feature_detec...