build: consolidate kldxref and move NO_XREF logic

[kevans]

Summary

The current application of NO_XREF works fine for normal module builds,
but falls apart quickly for LOCAL_MODULES. In particular, trying to
LOCAL_MODULES in the drm-kmod-firmware modules will often blow up
spectacularly on install as kldxref(8) races against install(1) and the
latter fails horribly when it encounters a module that hasn't finished
installing yet (at its INS@ temporary-file name).

Re-organizing the kldxref logic into kern.mk is maybe a little unusual
for a file that doesn't have any other targets, but it makes enough
sense to consolidate the two occurrences into one shared between the
kmod and kernel build that I think it's fine to push boundaries here.

[kevans]

Left out of the message: I suspect nobody is trying a full sys/modules build outside of installkernel / install from the config(8) generated target, whichever workflow you prefer, so I don't think we're hanging anyone out to dry by sys/modules not doing this anymore