Add an explicit std feature so that features are purely additive.

lib/native/src/lib.rs

@@ -1,8 +1,9 @@
 //! Performs autodetection of the host for the purposes of running
 //! Cretonne to generate code to run on the same machine.
-#![cfg_attr(feature = "no_std", no_std)]
 #![deny(missing_docs)]
 
+#![cfg_attr(not(feature = "std"), no_std)]
+
 extern crate cretonne;
 
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]