AArch64: combine arm64 & aarch64 cases in switch

There shouldn't be any difference in behaviour here, at least not in
any configurations people care about and possibly not in any reachable
configurations.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@209899 91177308-0d34-0410-b5e6-96231b3b80d8