stop_all_threads: (re-)enable async before waiting for stops

stop_all_threads: (re-)enable async before waiting for stops

Running the
gdb.threads/step-over-thread-exit-while-stop-all-threads.exp testcase
added later in the series against gdbserver, after the
TARGET_WAITKIND_NO_RESUMED fix from the following patch, would run
into an infinite loop in stop_all_threads, leading to a timeout:

  FAIL: gdb.threads/step-over-thread-exit-while-stop-all-threads.exp: displaced-stepping=off: target-non-stop=on: iter 0: continue (timeout)

The is really a latent bug, and it is about the fact that
stop_all_threads stops listening to events from a target as soon as it
sees a TARGET_WAITKIND_NO_RESUMED, ignoring that
TARGET_WAITKIND_NO_RESUMED may be delayed.  handle_no_resumed knows
how to handle delayed no-resumed events, but stop_all_threads was
never taught to.

In more detail, here's what happens with that testcase:

#1 - Multiple threads report breakpoint hits to gdb.

#2 - gdb picks one events, and it's for thread 1.  All other stops are
     left pending.  thread 1 needs to move past a breakpoint, so gdb
     stops all threads to start an inline step over for thread 1.
     While stopping threads, some of the threads that were still
     running report events that are also left pending.

#2 - gdb steps thread 1

#3 - Thread 1 exits while stepping (it steps over an exit syscall),
     gdbserver reports thread exit for thread 1

#4 - Thread 1 was the last resumed thread, so gdbserver also reports
     no-resumed:

    [remote]   Notification received: Stop:w0;p3445d0.3445d3
    [remote] Sending packet: $vStopped#55
    [remote] Packet received: N
    [remote] Sending packet: $vStopped#55
    [remote] Packet received: OK

#5 - gdb processes the thread exit for thread 1, finishes the step
     over and restarts threads.

#6 - gdb picks the next event to process out of one of the resumed
     threads with pending events:

    [infrun] random_resumed_with_pending_wait_status: Found 32 events, selecting #11

#7 - This is again a breakpoint hit and the breakpoint needs to be
     stepped over too, so gdb starts a step-over dance again.

#8 - We reach stop_all_threads, which finds that some threads need to
     be stopped.

#9 - wait_one finally consumes the no-resumed event queue by #4.
     Seeing this, wait_one disable target async, to stop listening for
     events out of the remote target.

#10 - We still haven't seen all the stops expected, so
      stop_all_threads tries another iteration.

#11 - Because the remote target is no longer async, and there are no
      other targets, wait_one return no-resumed immediately without
      polling the remote target.

#12 - We still haven't seen all the stops expected, so
      stop_all_threads tries another iteration.  goto #11, looping
      forever.

Fix this by explicitly enabling/re-enabling target async on targets
that can async, before waiting for stops.

Reviewed-By: Andrew Burgess <aburgess@redhat.com>
Change-Id: Ie3ffb0df89635585a6631aa842689cecc989e33f