[kernel] 对rt_tick变量增加volatile修饰，防止编译器优化时出现问题

src/clock.c

@@ -22,7 +22,7 @@
 #ifdef RT_USING_SMP
 #define rt_tick rt_cpu_index(0)->tick
 #else
-static rt_tick_t rt_tick = 0;
+static volatile rt_tick_t rt_tick = 0;
 #endif
 
 /**