Add macros simplifying the fake preload setup

This is in preparation for booting via loader(8). Lift these macros from arm64
so we don't need to worry about the size when inserting new elements. This
could have been done in r359673, but I didn't think I would be returning to
this function so soon.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D24910