[X86][BF16] Enable __bf16 for x86 targets.

X86 psABI has updated to support __bf16 type, the ABI of which is the
same as FP16. See https://discourse.llvm.org/t/patch-add-optional-bfloat16-support/63149

Reviewed By: pengfei

Differential Revision: https://reviews.llvm.org/D130964