syntax: Implement #![no_core]

This commit is an implementation of [RFC 1184][rfc] which tweaks the behavior of
the `#![no_std]` attribute and adds a new `#![no_core]` attribute. The
`#![no_std]` attribute now injects `extern crate core` at the top of the crate
as well as the libcore prelude into all modules (in the same manner as the
standard library's prelude). The `#![no_core]` attribute disables both std and
core injection.

[rfc]: https://github.com/rust-lang/rfcs/pull/1184

src/libstd/sys/unix/stack_overflow.rs

@@ -8,8 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#[cfg(stage0)]
+use core::prelude::v1::*;
+
 use libc;
-use core::prelude::*;
 use self::imp::{make_handler, drop_handler};
 
 pub use self::imp::{init, cleanup};