Remove unused fields from struct pcb.

[jhb]

Summary

cpu_switch/throw() and savectx() do not save or restore any values
in these fields which mostly held non-callee-save registers.

makectx() copied these fields from kdb_frame, but they weren't used
except for PC_REGS using pcb_sepc. Change PC_REGS to use
kdb_frame->tf_sepc directly instead.

Test Plan

• booted qemu, stack traces work for both curthread and other threads in ddb

[br]

Looks good