[Bug libstdc++/125890] allocator_traits<polymorphic_allocator> doesn't provide allocate_at_least in 16.1

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125890

--- Comment #2 from TC <rs2740 at gmail dot com> ---
(In reply to Drea Pinski from comment #1)
> I doubt that can be backported.

The whole patch may well be too much to backport, but the fix for this specific
problem is just a few self-contained lines.