rt_hw_cpu_shutdown: implement default weak function

and remvoe duplicated default functions in each cpu/bsp level

libcpu/unicore32/sep6200/cpu.c

@@ -260,7 +260,7 @@ void rt_hw_cpu_reset()
  *  shutdown CPU
  *
  */
-rt_weak void rt_hw_cpu_shutdown()
+void rt_hw_cpu_shutdown(void)
 {
     rt_base_t level;
     rt_kprintf("shutdown...\n");