libclang_rt: Descend into libclang_rt to clean

Otherwise, during a clean buildworld we don't descend into libclang_rt
to remove object files.

Reviewed by: brooks, dim, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D50766