This adds the linux native target non-stop mode support:

- Not stop all threads when a thread stops.

- Be sure we're not reading registers and memory from
  running threads.

- Add threads to the thread table as soon as we detect
  them.

- Avoid using ptrace on running threads.

- Implement target_stop_ptid to interrupt only
  one thread

- Getting the last pending event of a thread is
  different in nonstop due to the fact that
  stop_signal is per-thread in non-stop mode.

-- 
Pedro Alves