Handle integer-extending for C ABI

We need to supply sext/zext attributes to LLVM to ensure that arguments
are extended to the appropriate width in the correct way.

Most platforms extend integers less than 32 bits, though not all.

src/test/run-pass/cabi-int-widening.rs

@@ -0,0 +1,22 @@
+// Copyright 2016 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.
+
+#[link(name = "rust_test_helpers")]
+extern {
+    fn rust_int8_to_int32(_: i8) -> i32;
+}
+
+fn main() {
+    let x = unsafe {
+        rust_int8_to_int32(-1)
+    };
+
+    assert!(x == -1);
+}