core: fix cargo build for targets with "max-atomic-width": 0

This crate was failing to compile due to dead_code/unused_imports
warnings. This commits disables these two lints for these targets.

src/libcore/sync/atomic.rs

@@ -74,6 +74,8 @@
 //! ```
 
 #![stable(feature = "rust1", since = "1.0.0")]
+#![cfg_attr(not(target_has_atomic = "8"), allow(dead_code))]
+#![cfg_attr(not(target_has_atomic = "8"), allow(unused_imports))]
 
 use self::Ordering::*;