Cleanup old debug dirs in delete-old-dirs target

Any .debug or .symbols files under /usr/lib/debug which correspond to
OLD_FILES entries in ObsoleteFiles.inc are also automatically cleaned up
by the delete-old target. Make this also apply to any OLD_DIRS entries.

Reviewed by: emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D8683