[AArch64] Add ARMv8.2 command line option and feature flag.

[AArch64] Add ARMv8.2 command line option and feature flag.

ARMv8.2 is an architectural extension of ARMv8. This patch adds an
architecture feature macro for ARMv8.2 to the binutils AArch64 target
with GAS command line option -march=armv8.2-a.

gas/
2015-11-19  Matthew Wahab  <matthew.wahab@arm.com>

	* config/tc-aarch64.c (aarch64_archs): Add "armv8.2-a".
	* doc/c-aarch64.texi (-march): Likewise.

include/opcode/
2015-11-19  Matthew Wahab  <matthew.wahab@arm.com>

	* aarch64.h (AARCH64_FEATURE_V8_2): New.
	(AARCH64_ARCH_V8_2): New.

Change-Id: I129232ab00234a07d18ce4b619607344acb3cbaf