[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>