Add sys_yield tests and compile them to different location.

user/src/bin/02write_c.rs

@@ -0,0 +1,18 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+use user_lib::sys_yield;
+
+#[no_mangle]
+fn main() -> i32 {
+    for _ in 0..3 {
+        for _ in 0..10 { print!("C"); }
+        println!("");
+        sys_yield();
+    }
+    println!("Test write_c OK!");
+    0
+}