Add test for target_feature

This test checks that all of the x86 architectures (both `x86` and
`x86_64`) have the `sse2` feature. This is currently true for all of
the targets whose target CPU is `pentium4` (or better), but it might
fail on other targets (for example on `i586`).

src/test/run-pass/sse2.rs

@@ -0,0 +1,18 @@
+// Copyright 2012-2016 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.
+
+#![feature(cfg_target_feature)]
+
+pub fn main() {
+    if cfg!(any(target_arch = "x86", target_arch = "x86_64")) {
+        assert!(cfg!(target_feature = "sse2"),
+            "SSE2 was not detected as available on an x86 platform");
+    }
+}