添加用户系统调用sys get time

ch3/user/src/syscall.rs

@@ -3,6 +3,7 @@ use core::arch::asm;
 const SYSCALL_WRITE: usize = 64;
 const SYSCALL_EXIT: usize = 93;
 const SYSCALL_YIELD: usize = 124;
+const SYSCALL_GET_TIME: usize = 169;
 
 
 fn syscall(id: usize, args: [usize; 3]) -> isize {
@@ -33,3 +34,8 @@ pub fn sys_yield() {
     syscall(SYSCALL_YIELD, [0, 0, 0]);
 }
 
+pub fn sys_get_time() -> isize {
+    syscall(SYSCALL_GET_TIME, [0, 0, 0])
+}
+
+