Final bit of P0063 - make sure that aligned_alloc is available when the...

Final bit of P0063 - make sure that aligned_alloc is available when the underlying C library supports it

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@338457 91177308-0d34-0410-b5e6-96231b3b80d8