kgdb uses td_oncpu to determine if a thread is running and should use
a pcb from stoppcbs[] rather than the thread's PCB. However, exited threads
retained td_oncpu from the last time they ran, and newborn threads had their
CPU fields cleared to zero during fork and thread creation since they are
in the set of fields zeroed when threads are setup. To fix, explicitly
update the CPU fields for exiting threads in sched_throw() to reflect the
switch out and reset the CPU fields for new threads in sched_fork_thread()
to NOCPU.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D3193