[PATCH] [libcxx] Mark atomic tests UNSUPPORTED for c++03 and c++98.

[Dan Albert]

I'm saying we might want to verify that these do in fact fail. UNSUPPORTED
causes the test to not be run, XFAIL tests to ensure failure, which is
possibly what we want here. OTOH, our test configuration isn't set up to do
conditional _compile time_ failures, so maybe we should just settle for
UNSUPPORTED.

http://reviews.llvm.org/D5955