[AArch64] Add command-line options for ARMv8.2-A

This adds new values for the -march option (armv8.2a and armv8.2-a,
which are aliases of each other), and new suffixes for the -march and
-mcpu options (+fp16 and +nofp16), to allow targeting the ARMv8.2-A
architecture and it's optional half-precision floating-point extension.

Differential Revision: http://reviews.llvm.org/D15022

llvm-svn: 254160