Auto merge of #28612 - gandro:targetvendor, r=alexcrichton

This adds a new target property, `target_vendor`. It is to be be used as a matcher for conditional compilation. The vendor is part of the [autoconf target triple](http://llvm.org/docs/doxygen/html/classllvm_1_1Triple.html#details): `<arch><sub>-<vendor>-<os>-<env>`. `arch`, `target_os` and `target_env` are already supported by Rust.

This change was suggested in PR #28593. It enables conditional compilation based on the vendor. This is needed for the rumprun target, which needs to match against both, target_os and target_vendor.

The default value for `target_vendor` is "unknown", "apple" and "pc" are other common values.

Matching against the `target_vendor` is introduced behind the feature gate `#![feature(cfg_target_vendor)]`.

This is the first time I messed around with rustc internals. I just added the my code where I found the existing `target_*` variables, hopefully I haven't missed anything. Please review with care. :)

r? @alexcrichton

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() {
+}