kboot: Rework _start

Split _start into _start and _start_c (inspired by musl and the powerpc
impl is copied from there). This allows us to actually get the command
line arguments on all the platforms. We have a very simplified startup
that supports only static linking.

Sponsored by: Netflix

(cherry picked from commit f5ed1b0f84bf06839f0947981ebdb3646c3c433a)