Make kldxref a bootstrap tool and use unconditionally

Now that kldxref is a generic cross tool and can be built on non-FreeBSD
we can bootstrap it during the build and thus remove the condition for
whether it exists. We also need to make sure to add it to the METALOG
for -DNO_ROOT builds.

Reviewed by: brooks, imp
Differential Revision: https://reviews.freebsd.org/D43051