rt_hw_cpu_shutdown: implement default weak function

and remvoe duplicated default functions in each cpu/bsp level

libcpu/aarch64/common/cpu.c

@@ -357,7 +357,7 @@ const char *rt_hw_cpu_arch(void)
 }
 
 /** shutdown CPU */
-rt_weak void rt_hw_cpu_shutdown()
+void rt_hw_cpu_shutdown(void)
 {
     rt_uint32_t level;
     rt_kprintf("shutdown...\n");

libcpu/aarch64/common/cpu.h

@@ -56,8 +56,6 @@ extern struct cpu_ops_t cpu_ops_spin_tbl;
 
 #endif /* RT_USING_SMP */
 
-extern void rt_hw_cpu_shutdown(void);
-
 extern void (*system_off)(void);
 
 #endif /* __RT_HW_CPU_H__ */