Don't try and run kldxref for arm kernels

Surprisingly, kldxref does not currently support arm, and unhelpfully
this means it silently does nothing rather than give an error, so the
linker.hints entry added to the METALOG for -DNO_ROOT builds (and
pkgbase ones) refers to a file that doesn't exist. Ideally it would be
supported (and ideally the METALOG handling would be less fragile, but
without integrating it into kldxref the only real option would be to
just run find(1) to get the list of linker.hints files, which feels a
little backwards), but for now just paper over this by skipping the
build step on arm.

Reported by: bapt
Fixes: ff7c12c1f17e ("Make kldxref a bootstrap tool and use unconditionally")