The program counter field in the PCB is written in exactly one place,
makectx(), upon entry to the debugger. For threads other than
curthread, its value will be empty, or bogus. Rather than writing to
this field in more places, it can be removed in favor of using the value
in the link register.
To make this clearer, pcb->pcb_x is renamed to pcb->pcb_lr, similar
to what already exists in struct trapframe. Also, prefer lr to x30 in
assembly, as it better conveys intention.
This improves PC_REGS() for kdb_thread != curthread. It is required for
a functional gdb stub, fixing the output of info threads, in