Make kldxref a bootstrap tool and use unconditionally

[jrtc27]

Summary

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.

[brooks]

sys/conf/kmod.mk

365

Should this be indented 4-spaces instead of a tab as a shell continuation?

[jrtc27]

sys/conf/kmod.mk

365

Yeah, it should, I copied it from somewhere that got it wrong. Will fix when committing.