[X86] Support `_Float16` on SSE2 and up

This is split from D113107 to address #56204 and https://discourse.llvm.org/t/how-to-build-compiler-rt-for-new-x86-half-float-abi/63366

Reviewed By: zahiraam, rjmccall, bkramer

Differential Revision: https://reviews.llvm.org/D128571