[libcxx] Fix incorrect XFAILs for chrono tests on old macos deployment targets

The tests were marked to fail based on the 'availability' LIT feature.
However, those tests should really only be failing when we run them
against the dylibs that were deployed on macosx10.7 and macosx10.8,
which the deployment target has nothing to do with.

This caused the tests to unexpectedly pass when running the tests
with deployment target macosx10.{7,8} but running with a recent dylib.

llvm-svn: 348520