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_jemalloc/lib.rs

@@ -16,6 +16,7 @@
             reason = "this library is unlikely to be stabilized in its current \
                       form or name",
             issue = "27783")]
+#![cfg_attr(not(stage0), deny(warnings))]
 #![feature(allocator)]
 #![feature(libc)]
 #![feature(staged_api)]