Centralize and simpify implemention of some VM macros

These macros have substantially identical implementations on each
platform. Use roundup2/rounddown2 for round_page/trunc_page.

This version standardizes on not using explicit casts and instead
preserving the original type. A couple of tweaks were required to
make this work.

Reviewed by: brooks, kib, markj
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D48450