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

[Nick Clifton]

Hi Szabolcs,

> gas/
> 2016-12-05  Szabolcs Nagy  <szabolcs.nagy@arm.com>
> 
> 	* config/tc-arm.c (arm_archs): Add "armv8.3-a".
> 	* doc/c-arm.texi (-march): Add "armv8.3-a".
> 
> include/opcode/
> 2016-12-05  Szabolcs Nagy  <szabolcs.nagy@arm.com>
> 
> 	* arm.h (ARM_EXT2_V8_3A, ARM_AEXT2_V8_3A): New.
> 	(ARM_ARCH_V8_3A): New.

Approved - please apply.

Cheers
  Nick