[AArch64] Add feature flags and command line for ARMv8.2 FP16 support.

[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>

	* config/tc-aarch64.c (aarch64_features): Add "fp16".
	* doc/c-aarch64.texi (Architecture Extensions): Add "fp16".

include/opcode/
2015-11-27  Matthew Wahab  <matthew.wahab@arm.com>

	* aarch64.h (AARCH64_FEATURE_F16): New.
	(AARCH64_ARCH_V8_2): Add AARCH64_FEATURE_F16 to ARMv8.2
	features.

Change-Id: Id2021e0513946e16d0935c2a5b9605574cdff95a