test: fix test on Windows

When building on Windows, the default target is Windows.  Windows on ARM does
not support ARM mode compilation, resulting in test failures.  Simply specify a
triple to ensure that we are testing the correct behaviour.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208340 91177308-0d34-0410-b5e6-96231b3b80d8