[ARM/AArch64][v8.5A] Add Armv8.5-A target

This patch allows targetting Armv8.5-A from Clang. Most of the
implementation is in TargetParser, so this is mostly just adding tests.

Patch by Pablo Barrio!

Differential revision: https://reviews.llvm.org/D52491

llvm-svn: 343111