Add explicit triples to AArch64 tests

Only Linux is supported at the moment, and other platforms quickly fault. As a
result these tests would fail on non-Linux hosts. It may be worth making the
tests more generic again as more platforms are supported.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174170 91177308-0d34-0410-b5e6-96231b3b80d8