add weak memcpy et al symbols

closes #28

library/compiler-builtins/src/lib.rs

@@ -1,10 +1,12 @@
 #![allow(unused_features)]
 #![feature(asm)]
 #![feature(core_intrinsics)]
+#![feature(linkage)]
 #![feature(naked_functions)]
 #![cfg_attr(not(test), no_std)]
 // TODO(rust-lang/rust#35021) uncomment when that PR lands
 // #![feature(rustc_builtins)]
+#![no_builtins]
 
 // We disable #[no_mangle] for tests so that we can verify the test results
 // against the native compiler-rt implementations of the builtins.
@@ -20,6 +22,7 @@ extern crate core;
 pub mod arm;
 
 pub mod udiv;
+pub mod mem;
 pub mod mul;
 pub mod shift;