Use the new panic handler feature

The "panic_fmt" lang item no longer exists; it has been replaced with
the unstable feature #[panic_handler]

src/lib.rs

@@ -7,6 +7,7 @@
 #![feature(associated_type_defaults)]
 #![feature(const_fn)]
 #![feature(lang_items)]
+#![feature(panic_handler)]
 #![feature(unwind_attributes)]
 
 #![no_core]

src/std_stub.rs

@@ -7,9 +7,8 @@ pub mod std {
     pub unsafe extern "C" fn rust_eh_personality(_state: (), _exception_object: *mut (), _context: *mut ()) -> () {
     }
 
-    #[lang = "panic_fmt"]
-    #[unwind]
-    pub extern fn rust_begin_panic(_msg: (), _file: &'static str, _line: u32) -> ! {
+    #[panic_handler]
+    fn panic(_info: &::core::panic::PanicInfo) -> ! {
         loop {}
     }
 }