[AArch64] Add ARMv8.3 command line option and feature flag

[AArch64] Add ARMv8.3 command line option and feature flag

ARMv8.3 can be selected with -march=armv8.3-a command line option.
An overview of the ARMv8.3 architecture extension is at
https://community.arm.com/groups/processors/blog/2016/10/27/armv8-a-architecture-2016-additions

gas/
2016-11-11  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	* config/tc-aarch64.c (aarch64_archs): Add "armv8.3-a".
	* doc/c-aarch64.texi (-march): Likewise.

include/
2016-11-11  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	* opcode/aarch64.h (AARCH64_FEATURE_V8_3): Define.
	(AARCH64_ARCH_V8_3): Define.
	(AARCH64_ARCH_V8_1, AARCH64_ARCH_V8_2): Simplify.