Fix a test that I broke over the weekend

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@195143 91177308-0d34-0410-b5e6-96231b3b80d8
Patch by Xing Xue to improve libc++ support for AIX

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@195144 91177308-0d34-0410-b5e6-96231b3b80d8