[ARM/AArch64] Support FP16 +fp16fml instructions

Add +fp16fml feature for new FP16 instructions, which are a
mandatory part of FP16 from v8.4-A and an optional part of FP16
from v8.2-A. It doesn't seem to be possible to model this in
LLVM, but the relationship between the options is handled by
the related clang patch.

In keeping with what I think is the usual practice, the fp16fml
extension is accepted regardless of base architecture version.

Builds on/replaces Sjoerd Meijer's patch to add these instructions at
https://reviews.llvm.org/D49839.

Differential Revision: https://reviews.llvm.org/D50228

llvm-svn: 340013