Auto merge of #38079 - BurntSushi:attrtarget, r=alexcrichton

Add new #[target_feature = "..."] attribute.

This commit adds a new attribute that instructs the compiler to emit
target specific code for a single function. For example, the following
function is permitted to use instructions that are part of SSE 4.2:

    #[target_feature = "+sse4.2"]
    fn foo() { ... }

In particular, use of this attribute does not require setting the
-C target-feature or -C target-cpu options on rustc.

This attribute does not have any protections built into it. For example,
nothing stops one from calling the above `foo` function on hosts without
SSE 4.2 support. Doing so may result in a SIGILL.

I've also expanded the x86 target feature whitelist.

src/test/compile-fail/gated-target_feature.rs

@@ -0,0 +1,13 @@
+// Copyright 2015 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.
+
+#[target_feature = "+sse2"]
+//~^ the `#[target_feature]` attribute is an experimental feature
+fn foo() {}