[ARM] Add command-line options for ARMv8.2-A
This allows ARMv8.2-A to be targeted either by using "armv8.2a" in the
triple, or by using -march=armv8.2-a (or the alias -march=armv8.2a).
The FP16 extension can be enabled with the "+fp16" suffix to the -march
or -mcpu option. This is consistent with the AArch64 option, rather than
the usual ARM option of -mfpu. We have agreed with the team which will
be upstreaming this to GCC that we want to use this new option format
for new architecture extensions for both ARM and AArch64.
Most of the work for this was done by the TargetParser patch in llvm.
Differential Revision: http://reviews.llvm.org/D15040