make showconfig: set MACHINE appropriately

[emaste]

Summary

Previously MACHINE was not set for the showconfig target, and was 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
Sponsored by: The FreeBSD Foundation

[emaste]

Makefile.inc1

3380

Perhaps MACHINE_ARCH=${TARGET_ARCH} too? I notice builddtb below sets only MACHINE.

[imp]

Don't see that MACHINE_ARCH is used, but it's always a good idea to set MACHINE and MACHINE_ARCH in pairs.