Link test to compiler builtins and make unstable

This commit fixes a test which now needs to explicitly link to the
`compiler_builtins` crate as well as makes the `compiler_builtins` crate
unstable.

src/libstd/lib.rs

@@ -224,6 +224,7 @@
 #![feature(char_internals)]
 #![feature(collections)]
 #![feature(collections_bound)]
+#![feature(compiler_builtins_lib)]
 #![feature(const_fn)]
 #![feature(core_float)]
 #![feature(core_intrinsics)]