[X86][RFC] Enable `_Float16` type support on X86 following the psABI

GCC and Clang/LLVM will support `_Float16` on X86 in C/C++, following
the latest X86 psABI. (https://gitlab.com/x86-psABIs)

_Float16 arithmetic will be performed using native half-precision. If
native arithmetic instructions are not available, it will be performed
at a higher precision (currently always float) and then truncated down
to _Float16 immediately after each single arithmetic operation.

Reviewed By: LuoYuanke

Differential Revision: https://reviews.llvm.org/D107082