XFAIL this test when using the darwin12 system library. Reviewed by Howard

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@181610 91177308-0d34-0410-b5e6-96231b3b80d8
İsmail Dönmez: Enable quick_exit on linux.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@181612 91177308-0d34-0410-b5e6-96231b3b80d8