Use the context created in makectx() for stack traces.

Always use the kdb_thr_ctx() for db_trace_thread() as on other
architectures. Initialize pcb_ra to be the sepc from the saved
trapframe rather than the saved ra to avoid skipping a frame.

Reviewed by: mhorne, br
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D23513