kboot: Add parsing of /proc/iomem into seg.c

We'll be using this code for most / all of the platforms since iomem is
the only interface that can tell us of the reserved to the linux kernel
areas that we cannot place the new kernel into, but that we are free to
use once we hit trampoline. aarch64 will use this shortly, and similar
code in amd64 will be refactored when I make that platform work.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D38309