[AArch64] Add feature flags and command line for ARMv8.2 FP16 support.
ARMv8.2 adds optional support for 16-bit operations to the FP and
Adv.SIMD instructions. This patch adds a feature macro for this support
with a new command line option "+fp16" to enable/disable it.
Although the command line option is added as an architecture extension,
it only affects instructions available with when +fp or +simd is
enabled. If +fp16 is specified then it will also enable +fp.
There are currently no FP16 instructions implemented in binutils, this
patch is to enable subsequent work on supporting the extension.
gas/
2015-11-27 Matthew Wahab <matthew.wahab@arm.com>