[Bug libstdc++/96942] std::pmr::monotonic_buffer_resource causes CPU cache misses

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96942

--- Comment #19 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The upstream isn't necessarily malloc (although we could add a check to find
out whether it is the std::new_delete_resource() if we wanted to).

Suggestions for improvement are welcome. This is only the second time I've ever
received any indication anybody is even using the <memory_resource> header, so
I've not wasted my time tuning it.