[libcxx] Only enable the availability LIT feature when we're testing libc++

Other standard libraries don't implement availability markup, so it doesn't
make sense to e.g. XFAIL tests based on availability markup outside of
libc++.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@348871 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed