stand: userboot: allow building on !x86

We can still get plenty of use out of a userboot that doesn't know
anything about how to load or boot a kernel; notably, the test harness
in tools/boot can still be used to test lua changes.

Hack out the necessary bits to simply build on other platforms, and add
a small warning with ample time to view the warning on other platforms.
We still won't build userboot by default on these platforms, since the
build product isn't useful for most people.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D41529