[ARM][AArch64] ARMv8.3-A enablement

The beta ARMv8.3 ISA specifications have been released for AArch64
and AArch32, these can be found at:
https://developer.arm.com/products/architecture/a-profile/exploration-tools

An introduction to this architecture update can be found at:
https://community.arm.com/processors/b/blog/posts/armv8-a-architecture-2016-additions

This patch is the first in a series which will add ARM v8.3-A support
in LLVM and Clang. It adds the necessary changes that create targets
for both the ARM and AArch64 backends.

Differential Revision: https://reviews.llvm.org/D36514

llvm-svn: 310561