AArch64: set all processor features from -arch if nothing else present

Darwin's "-arch arm64" option implies full Cyclone CPU, for both architectural
and tuning purposes. So if neither of the explicit options have been given,
forward that on to the proper invocation.

rdar://problem/18906227

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221631 91177308-0d34-0410-b5e6-96231b3b80d8