MFC 357305: Remove unused fields from struct pcb.

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.