Patch: support callback on allocation overflow instead of calling abort()

[Colin Caine]

Sorry, this patch was posted to this list last month, but I didn't see it
in the archives. See here for related discussion:
https://gmplib.org/list-archives/gmp-bugs/2020-September/004865.html

Though this discussion chain about memory leaks is new :)

It's worth noting that this patch is useful to Julia even with memory leaks
(Julia does not patch the leaks out afaict).