arm64: improve UVA layout for 32bit processes

Add compat.aarch32 tunables for maxssiz, maxdsiz, and maxvmem.
Set the default values same as for amd64.
Fix freebsd32 sysentvec on arm64 to provide sv_maxssiz, and sv_fixlimit.

PR: 274705
Reviewed by: markj
Tested by: fuz
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D42451