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/libsyntax_ext/lib.rs

@@ -11,13 +11,13 @@
 //! Syntax extensions in the Rust compiler.
 
 #![crate_name = "syntax_ext"]
+#![unstable(feature = "rustc_private", issue = "27812")]
 #![crate_type = "dylib"]
 #![crate_type = "rlib"]
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "https://doc.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/nightly/")]
-
-#![unstable(feature = "rustc_private", issue = "27812")]
+#![cfg_attr(not(stage0), deny(warnings))]
 
 #![feature(rustc_private)]
 #![feature(staged_api)]