arm64: update kdb_thrctx->pcb_lr with BKPT_SKIP

This value should be kept in sync with updates to kdb_frame->tf_elr,
since it is queried by PC_REGS() in several places.

Reviewed by: markj, jhb
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28943