Added backtrace function

Added PX4_BACKTRACE() to display stack trace for debugging

Added backtrace for HRT reschedule with 0 timeout

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/platforms/posix/work_queue/hrt_thread.c

@@ -165,6 +165,7 @@ static void hrt_work_process()
 
 			if (!worker) {
 				PX4_ERR("MESSED UP: worker = 0");
+				PX4_BACKTRACE();
 
 			} else {
 				worker(arg);