Handle non-page aligned/sized memory in physmem

In some configurations the firmware may pass memory regions that are
not page sized or aligned, e.g. when using 16k pages on arm64. If this
is the case we will calculate many small regions because the alignment
is applied before being inserted. As we round the start up and end down
this will leave a 1 page hole between what should have been a single
region.

Fix by keeping the original alignment until we are just about to insert
the region into the avail array.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34694