MachO: use "arm64" as the triple name in modules.

Current versions of ld64 can't cope with "aarch64" being stored. I'm fixing
that, but in the transitionary period we'll need to still emit "arm64".

rdar://problem/17783765

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@213852 91177308-0d34-0410-b5e6-96231b3b80d8