Fixed atomic access to flags in sys.execute.

This seems to fix the bug that caused Grbl to hang during some operations,
especially jogging.

probe.c

@@ -45,7 +45,7 @@ void probe_state_monitor()
     if (probe_get_state()) {
       sys.probe_state = PROBE_OFF;
       memcpy(sys.probe_position, sys.position, sizeof(float)*N_AXIS);
-      sys.execute |= EXEC_FEED_HOLD;
+      bit_true(sys.execute, EXEC_FEED_HOLD);
     }
   }
 }