drop compiler-builtins bits

they were leftover from rebasing

closes #31

Author: japaric

cortex-m-rt/src/lib.rs

@@ -275,16 +275,14 @@
 #![deny(missing_docs)]
 #![deny(warnings)]
 #![feature(asm)]
-#![cfg_attr(needs_cb, feature(compiler_builtins_lib))]
+#![feature(core_intrinsics)]
 #![feature(global_asm)]
 #![feature(lang_items)]
 #![feature(linkage)]
 #![feature(naked_functions)]
 #![feature(used)]
 #![no_std]
 
-#[cfg(needs_cb)]
-extern crate compiler_builtins;
 #[cfg(target_arch = "arm")]
 extern crate cortex_m;
 #[cfg(target_arch = "arm")]