[libcxx] [test] Fix experimental/memory.resource.adaptor.mem/db_deallocate on Windows

The checks within the libc++experimental memory_resource class uses this
limit:

     _MaxAlign = _LIBCPP_ALIGNOF(max_align_t);

Therefore, only use max_align_t for this limit instead of using
`__STDCPP_DEFAULT_NEW_ALIGNMENT__` if available.

Differential Revision: https://reviews.llvm.org/D105905