AArch64: re-enable tests that were looking for a non-existent backend.

In the final phase of the merge, I managed to disable a bunch of Clang
tests accidentally. Fortunately none of them seem to have broken in
the interim.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@211149 91177308-0d34-0410-b5e6-96231b3b80d8