[libcxx] Properly mark aligned allocation macro test as XFAIL on OS X

This test was initially marked as XFAIL using `XFAIL: macosx10.YY`, and
was then moved to `UNSUPPORTED: macosx10.YY`. The intent is to mark the
test as XFAILing when a deployment target older than macosx10.14 is used,
and the right way to do this is `XFAIL: availability=macosx10.YY`.

llvm-svn: 349426