mk: Move from -D warnings to #![deny(warnings)]

This commit removes the `-D warnings` flag being passed through the makefiles to
all crates to instead be a crate attribute. We want these attributes always
applied for all our standard builds, and this is more amenable to Cargo-based
builds as well.

Note that all `deny(warnings)` attributes are gated with a `cfg(stage0)`
attribute currently to match the same semantics we have today

src/libstd/lib.rs

@@ -273,6 +273,7 @@
 
 #![deny(missing_docs)]
 #![allow(unused_features)] // std may use features in a platform-specific way
+#![cfg_attr(not(stage0), deny(warnings))]
 
 #[cfg(test)] extern crate test;
 #[cfg(test)] #[macro_use] extern crate log;