make showconfig: set MACHINE and MACHINE_ARCH

Previously these were not set for the showconfig target, so took the
host's value rather than the target. As a result some machine-dependent
kernel options were not reported with correct defaults in the src.conf
manpage.

PR: 269994
Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38926