[libcxx] [modules] Add _LIBCPP_USING_IF_EXISTS on aligned_alloc (#89827)

This is missing e.g. on Windows. With this change, it's possible to make
the libcxx std module work on mingw-w64 (although that requires a few
fixes to those headers).

In the regular cstdlib header, we have _LIBCPP_USING_IF_EXISTS flagged
on every single reexported function (since
a9c9183c), but the modules seem to only
have _LIBCPP_USING_IF_EXISTS set on a few individual functions, so far.