kboot: Centralize ldscript addition

Make the pattern for ldscripts always be
arch/$MACHINE_ARCH/$MACHINE_ARCH.ldscript so we can add it from a
central Makefile. This also moves from ldscript.arch to arch.ldscript to
match the loader's new convention.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D44057