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
Sponsored by: The FreeBSD Foundation

(cherry picked from commit bd0b7cbf5ac1baff4211e09584e0b86d4d96228b)