[libcxxabi] Disable cxa_thread_atexit_test if unavailable

The feature check is already in place when building the library but wasn't
honored for the tests.

Differential Revision: http://reviews.llvm.org/D18205

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@263699 91177308-0d34-0410-b5e6-96231b3b80d8