ARMV8-A archkind and target defines helper functions

This introduces helper functions that set target defines for different ARMV8-A
architecture kinds. It fixes an issue that the v8.1 define ARM_FEATURE_QRDMX
was not set for v8.2. These helper functions make things more “scalable” if we
want to add ARMv8.3 at some point, and a cleanup has been done to hold the
architecture kind in one variable (instead of one for each).

Differential Revision: https://reviews.llvm.org/D34686

llvm-svn: 306805