kmod.mk: Allow extra objects to be specified in modules

OBJS are automatically added to CLEANFILES. For pre-built objects, this
is not desirable since it will delete the object from the source
tree. Introduce EXTRA_OBJS which list these object files, but aren't
added to clean files.

Sponsored by: Netflix
Reviewed by: emaste@
Differential Revision: https://reviews.freebsd.org/D30615