sys_yield tests worked on qemu.

os/src/task/task.rs

@@ -0,0 +1,18 @@
+pub struct TaskControlBlock {
+    pub task_cx_ptr: usize,
+    pub task_status: TaskStatus,
+}
+
+impl TaskControlBlock {
+    pub fn get_task_cx_ptr2(&self) -> *const usize {
+        &self.task_cx_ptr as *const usize
+    }
+}
+
+#[derive(Copy, Clone, PartialEq)]
+pub enum TaskStatus {
+    UnInit,
+    Ready,
+    Running,
+    Exited,
+}