The standard library doesn't depend on rustc_bitflags, so move it to explicit
dependencies on all other crates. Additionally, the arena/fmt_macros deps could
be dropped from libsyntax.
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
