kboot: MI fixups to enable aarch64 booting

A number of bug fixes to loading kernels and modules on aarch64 and amd64.
Fix offset calcuations.
Add a number of debugs, commented out for now (will GC them in the future)

With this, and the MD aarch64 commands, we can linux boot in qemu and on
real hardware.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D38261