test: Check native main() signature

src/test/codegen/abi-main-signature.rs

+// 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.
+
+// Checks the signature of the implicitly generated native main()
+// entry point. It must match C's `int main(int, char **)`.
+
+fn main() {
+}
+
+// CHECK: define i32 @main(i32, i8**)