kboot: Move _start out of kboot and into libkboot

Move the startup code from kboot/kboot to kboot/libkboot and add the
necessary infrastructure for it to build. move start_arch.h, a private
header for libkboot, over as well.

Sponsored by: Netflix

(cherry picked from commit a03411e84728e9b267056fd31c7d1d9d1dc1b01e)