Expose stopped cpu contexts to ddb on PowerPC

Summary: In r220638, stoppcbs started being tracked. This never got exposed to
ddb though, so kdb_thr_ctx() didn't know how to look them up.

This allows switching to threads on stopped CPUs in kdb.

Submitted by: Brandon Bergren <git_bdragon.rkt0.net>
Differential Revision: https://reviews.freebsd.org/D15986