init: cap -maxmempool and -dbcache on 32-bit systems

Summary:
32-bit architecture is limited to 4GiB, so it doesn't make sense to set a too high value. 500 MB is
chosen as an arbitrary maximum value that seems reasonable for maxmempool.

Since -dbcache is performance critical, pick an arbitrary value higher than for -maxmempool but still reasonable.

This is a partial backport of core#32530

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18843