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/liballoc_system/lib.rs

@@ -12,6 +12,7 @@
 #![crate_type = "rlib"]
 #![no_std]
 #![allocator]
+#![cfg_attr(not(stage0), deny(warnings))]
 #![unstable(feature = "alloc_system",
             reason = "this library is unlikely to be stabilized in its current \
                       form or name",