[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
a9c9183), but the modules seem to only
have _LIBCPP_USING_IF_EXISTS set on a few individual functions, so far.

libcxx/modules/std.compat/cstdlib.inc

@@ -25,7 +25,7 @@ export {
   using ::system;
 
   // [c.malloc], C library memory allocation
-  using ::aligned_alloc;
+  using ::aligned_alloc _LIBCPP_USING_IF_EXISTS;
   using ::calloc;
   using ::free;
   using ::malloc;