[AArch64] Alter arm_fp16.h to be target-based, not preprocessor based.

As the other recent patches, this alters the arm_fp16 intrinsics to be
target based, not preprocessor based. Apparently arm_fp16.h is AArch64
only under clang, making this mostly trivial with the TargetGuard
infrastructure.

Differential Revision: https://reviews.llvm.org/D137256