This patch fixes a GDBserver crash when one thread is stepping over
a syscall instruction which is exit.  Step-over isn't finished due
to the exit, but GDBserver doesn't clean up the state of step-over,
so in the wait next time, GDBserver will wait on step_over_bkpt,
which is already exited, and GDBserver crashes because
'requested_child' is NULL.  See gdbserver logs below,

Need step over [LWP 14858]? yes, found breakpoint at 0x2aaaaad91307^M
proceed_all_lwps: found thread 14858 needing a step-over^M
Starting step-over on LWP 14858.  Stopping all threads^M
>>>> entering void stop_all_lwps(int, lwp_info*)
....
<<<< exiting void stop_all_lwps(int, lwp_info*)^M
Done stopping all threads for step-over.^M
pc is 0x2aaaaad91307^M
Writing 0f to 0x2aaaaad91307 in process 14858^M
Could not find fast tracepoint jump at 0x2aaaaad91307 in list (uninserting).^M
  pending reinsert at 0x2aaaaad91307^M
  step from pc 0x2aaaaad91307^M
Resuming lwp 14858 (step, signal 0, stop not expected)^M

 # Start step-over for LWP 14858

>>>> entering ptid_t linux_wait_1(ptid_t, target_waitstatus*, int)
....
LLFE: 14858 exited.
...
<<<< exiting ptid_t linux_wait_1(ptid_t, target_waitstatus*, int)

  # LWP 14858 exited
.....
>>>> entering ptid_t linux_wait_1(ptid_t, target_waitstatus*, int)^M
linux_wait_1: [<all threads>]^M
step_over_bkpt set [LWP 14858.14858], doing a blocking wait

  # but step_over_bkpt is still LWP 14858, which is wrong

The fix is to finish step-over if it is ongoing, and unsuspend other
threads.  Without the fix in linux-low.c, GDBserver will crash in
with running gdb.base/step-over-exit.exp.

gdb/gdbserver:

2016-06-17  Yao Qi  <yao.qi@linaro.org>

	* linux-low.c (unsuspend_all_lwps): Declare.
	(linux_low_filter_event): If thread exited, call finish_step_over.
	If step-over is finished, unsuspend other threads.

gdb/testsuite:

2016-06-17  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/step-over-exit.c: New.
	* gdb.base/step-over-exit.exp: New.