Add mach build-stable to build with stable rustc

Github issue: #11806
Building with current stable rust (1.9.0) still fails because of
feature pragmas in some dependencies (e.g. serde_item).

ports/stable-rust/.gitignore

@@ -0,0 +1 @@
+/target

ports/stable-rust/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "stable-rust"
+version = "0.0.1"
+authors = ["The Servo Project Developers"]
+
+[dependencies]

ports/stable-rust/src/lib.rs

@@ -0,0 +1,10 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#[cfg(test)]
+mod test {
+    #[test]
+    fn it_works() {
+    }
+}