AArch64: add architecture version feature to Clang invocation.

Otherwise we don't get the correct predefines and so on in the front-end (or
the right features in the backend).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@289692 91177308-0d34-0410-b5e6-96231b3b80d8