kboot: Move load address stuff to MD code

The load address computations are highly architecture specific. There
are generic ways that are augmented by specific constraints of specific
way things work on each architecture. Move the current load segment
computations into a MD routine load_addr.

As part of the move, I'm marking kboot_get_kernel_machine_bits as
unused. This arrived in a prior commit, but never seems to have been
connected, suggesting an incomplete merge at the time, or a path not yet
taken.

Create a stub for amd64 that will be filled in with a later commit.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D36603