librustc: Put #[unsafe_destructor] behind a feature gate.

Closes #8142.

This is not the semantics we want long-term. You can continue to use
`#[unsafe_destructor]`, but you'll need to add
`#![feature(unsafe_destructor)]` to the crate attributes.

[breaking-change]

src/libcore/lib.rs

@@ -55,8 +55,9 @@
        html_playground_url = "http://play.rust-lang.org/")]
 
 #![no_std]
-#![feature(globs, macro_rules, managed_boxes, phase, simd)]
+#![feature(globs, macro_rules, managed_boxes, phase, simd, unsafe_destructor)]
 #![deny(missing_doc)]
+#![allow(unknown_features)] // NOTE: remove after a stage0 snap
 
 #[cfg(test)] extern crate realcore = "core";
 #[cfg(test)] extern crate libc;