Stage2: multiple user threads based on uniprocessor, see new added test race_adder and threads.

user/src/lib.rs

@@ -106,3 +106,14 @@ pub fn sleep(period_ms: usize) {
         sys_yield();
     }
 }
+
+pub fn thread_create(entry: usize) -> isize { sys_thread_create(entry) }
+pub fn gettid() -> isize { sys_gettid() }
+pub fn waittid(tid: usize) -> isize {
+    loop {
+        match sys_waittid(tid) {
+            -2 => { yield_(); }
+            exit_code => return exit_code,
+        }
+    }
+}