test: XFAIL the non-darwin gmlt test on darwin

r218702 disabled a -gmlt optimization for darwin, but this means the
non-darwin test isn't working there anymore.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218742 91177308-0d34-0410-b5e6-96231b3b80d8