Return event_ptid from ravenscar_thread_target::wait

ravenscar_thread_target::wait should return the event ptid from the
wrapped "wait" call in the situation where returning the Ravenscar
thread ptid is not appropriate.  This probably does not really make a
difference in practice, but it seemed like a reasonable cleanup.

gdb/ChangeLog
2020-08-07  Tom Tromey  <tromey@adacore.com>

	* ravenscar-thread.c (ravenscar_thread_target::wait): Return
	event_ptid.