rustc: Add target_vendor for target triples

This adds a new target property, `target_vendor` which can be used as a
matcher for conditional compilation. The vendor is part of the autoconf
target triple: <arch><sub>-<vendor>-<os>-<env>

The default value for `target_vendor` is "unknown".

Matching against the `target_vendor` with `#[cfg]` is currently feature
gated as `cfg_target_vendor`.

src/test/compile-fail/feature-gate-cfg-target-vendor.rs

@@ -0,0 +1,21 @@
+// 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.
+
+#[cfg(target_vendor = "x")] //~ ERROR `cfg(target_vendor)` is experimental
+#[cfg_attr(target_vendor = "x", x)] //~ ERROR `cfg(target_vendor)` is experimental
+struct Foo(u64, u64);
+
+#[cfg(not(any(all(target_vendor = "x"))))] //~ ERROR `cfg(target_vendor)` is experimental
+fn foo() {}
+
+fn main() {
+    cfg!(target_vendor = "x");
+    //~^ ERROR `cfg(target_vendor)` is experimental and subject to change
+}

src/test/run-pass/cfg-target-vendor.rs

@@ -0,0 +1,19 @@
+// 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.
+
+#![feature(cfg_target_vendor)]
+
+#[cfg(target_vendor = "unknown")]
+pub fn main() {
+}
+
+#[cfg(not(target_vendor = "unknown"))]
+pub fn main() {
+}