gdbserver/linux-low.cc: Ignore event_ptid if TARGET_WAITKIND_IGNORE

gdbserver/linux-low.cc: Ignore event_ptid if TARGET_WAITKIND_IGNORE

gdbserver's linux_process_target::wait loops if:

 - called in sync mode, and,
 - wait_1 returns TARGET_WAITKIND_IGNORE, _and_,
 - wait_1 also returns null_ptid.

The null_ptid check fails however when this path is taken:

   ptid_t
   linux_process_target::filter_exit_event (lwp_info *event_child,
					    target_waitstatus *ourstatus)
   {
   ...
     if (!is_leader (thread))
       {
	 if (report_exit_events_for (thread))
	   ourstatus->set_thread_exited (0);
	 else
	   ourstatus->set_ignore ();            <<<<<<<

	 delete_lwp (event_child);
       }
     return ptid;
   }

This makes linux_process_target::wait return TARGET_WAITKIND_IGNORE in
sync mode, which is unexpected by the core and fails an assertion.

This commit fixes it by just making linux_process_target::wait loop if
it got a TARGET_WAITKIND_IGNORE, irrespective of event_ptid.

Change-Id: I39776908a6c75cbd68aa04139ffcf7be334868cf