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