X86: Make specifying avx2 simpler on Darwin with '-arch'

Teach the '-arch' command line option to enable the compiler-friendly
features of core-avx2 CPUs on Darwin. Pass the information along in the
target triple like Darwin+ARM does.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194907 91177308-0d34-0410-b5e6-96231b3b80d8