Add first user program hello_world.rs

user/src/lang_items.rs

@@ -0,0 +1,10 @@
+#[panic_handler]
+fn panic_handler(panic_info: &core::panic::PanicInfo) -> ! {
+    let err = panic_info.message().unwrap();
+    if let Some(location) = panic_info.location() {
+        println!("Panicked at {}:{}, {}", location.file(), location.line(), err);
+    } else {
+        println!("Panicked: {}", err);
+    }
+    loop {}
+}