add unwind_attributes feature

Author: nikomatsakis

src/test/run-pass/abort-on-c-abi.rs

@@ -14,6 +14,8 @@
 // ignore-cloudabi no env and process
 // ignore-emscripten no processes
 
+#![feature(unwind_attributes)]
+
 use std::{env, panic};
 use std::io::prelude::*;
 use std::io;