AArch64: stop trying to take control of all UnknownArch triples.

This short-circuited our error reporting for incorrectly specified
target triples (you'd get AArch64 code instead).

Should fix PR20567.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215191 91177308-0d34-0410-b5e6-96231b3b80d8