arm: tune vmparam.h towards a little more modern

An 8MB max stack size is quite limiting in today's world, and in-fact is
the *default* stack size for almost every other arch (including mips).

Raise the default to 4MB (should be pretty reasonable) and the max to 64MB.
NetBSD made a similar move back in 2015 and raised MAXDSIZ to 1856 at the
same time, so let's just roll that in as well. They later lowered it, but
eventually raised it back to 1856 in order to build rust.

This was noticed while looking at qemu-bsd-user's default stack sizes and
growth behavior (or lack thereof).

Reviewed by: ian
Differential Revision: https://reviews.freebsd.org/D27218