add cfail test for the new feature gate

src/test/compile-fail/feature-gate-abi-msp430-interrupt.rs

@@ -0,0 +1,19 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that the MSP430 interrupt ABI cannot be used when msp430_interrupt
+// feature gate is not used.
+
+extern "msp430-interrupt" fn foo() {}
+//~^ ERROR msp430-interrupt ABI is experimental and subject to change
+
+fn main() {
+    foo();
+}