Fix XFAIL condition from r208840

The XFAILs in r208840 were too general. They were meant to only apply
to testing when use_system_lib was set.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@209972 91177308-0d34-0410-b5e6-96231b3b80d8