Switch ok with debug mode apps, implement sys_exit correctly later.

os/src/task/context.rs

@@ -0,0 +1,16 @@
+#[repr(C)]
+pub struct TaskContext {
+    ra: usize,
+    s: [usize; 12],
+}
+
+impl TaskContext {
+    pub fn goto_restore() -> Self {
+        extern "C" { fn __restore(); }
+        Self {
+            ra: __restore as usize,
+            s: [0; 12],
+        }
+    }
+}
+