Fix oversubscription with CHPL_LOCALE_MODEL=gpu (#26059)

The PRs to add GPU support to co-locales (PRs
#25734 and
#25846) broke oversubscription
such that no locales had any GPUs. This PR fixes that problem, and
cleans up resource allocation with co-locales in general.
Oversubscription is handled more cleanly, as is the "remainder" node
that occurs when the number of locales is not evenly divisible by the
number of nodes.

[Reviewed by @jabraham17, thank you.]