bsd.subdir.mk: Handle cleanobj.

Before this, the target was unknown. Now it will recurse on subdirs and run
the target in the current directory. It is required to recurse as there
may be subdirs that have objs in their directory or in the object directory,
so it is not enough to just delete the objdir of the subdir parent.

MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division