[ARM][AArch64] Add feature +fp16fml

Armv8.4-A adds a few FP16 instructions that can optionally be implemented
in CPUs of Armv8.2-A and above.

This patch adds a feature to clang to permit selection of these
instructions. This interacts with the +fp16 option as follows:

Prior to Armv8.4-A:
*) +fp16fml implies +fp16
*) +nofp16 implies +nofp16fml

From Armv8.4-A:
*) The above conditions apply, additionally: +fp16 implies +fp16fml

Patch by Bernard Ogden.

Differential Revision: https://reviews.llvm.org/D50229

llvm-svn: 342862