Add __attribute__((used)) so that the delay implementation doesn't get

optimized away as unreferenced, causing linker errors when trying to
resolve the weak reference to the missing function.

sys/arm/arm/mpcore_timer.c

@@ -370,6 +370,7 @@ DRIVER_MODULE(mp_tmr, simplebus, arm_tmr_driver, arm_tmr_devclass, 0, 0);
  *	nothing
  */
 static void
+__attribute__((used)) /* Must emit function code for the weak ref below. */
 arm_tmr_DELAY(int usec)
 {
 	int32_t counts_per_usec;