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

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

ARMv8.3 is an architectural extension of ARMv8.  Add the
feature macro and -march=armv8.3-a gas command line option
for the ARM target.

https://community.arm.com/groups/processors/blog/2016/10/27/armv8-a-architecture-2016-additions

gas/
	* config/tc-arm.c (arm_archs): Add "armv8.3-a".
	* doc/c-arm.texi (-march): Add "armv8.3-a".

include/
	* opcode/arm.h (ARM_EXT2_V8_3A, ARM_AEXT2_V8_3A): New.
	(ARM_ARCH_V8_3A): New.