[libcxx] Fix XFAIL for aligned deallocation test with trunk Clang

The test was marked as failing whenever the deployment target was 10.12
or older, but in reality the test passes when the deployment target is
10.12 on recent Clangs. This happens because only older clangs do not
honor the -faligned-allocation flag, which disables any availability
error related to aligned allocation support, regardless of the
deployment target.

llvm-svn: 347580