[risc-v][common64] Make rt_hw_cpu_shutdown weak to fix multiple definition error

Co-authored-by: BernardXiong <1241087+BernardXiong@users.noreply.github.com>

libcpu/risc-v/common64/cpuport.c

@@ -158,7 +158,7 @@ void rt_hw_context_switch_interrupt(void *context, rt_ubase_t from, rt_ubase_t t
 #endif /* end of RT_USING_SMP */
 
 /** shutdown CPU */
-void rt_hw_cpu_shutdown(void)
+rt_weak void rt_hw_cpu_shutdown(void)
 {
     rt_uint32_t level;
     rt_kprintf("shutdown...\n");