Use PCB/LR from PCB rather from stack on armv7-gdb

The kernel dump does not store these values on the stack.
Use PCB structure to resolve PC and LR properly.

Submitted by: Wojciech Macek <wma@semihalf.com>
Reviewed by: jhb, kib
Obtained from: Semihalf
Sponsored by: Juniper Networks Inc.
Differential Revision: https://reviews.freebsd.org/D4013