Auto merge of #7229 - servo:gold, r=metajack

Use the gold linker when available.

This shaves 50 seconds off the build time on my machine.

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/7229)
<!-- Reviewable:end -->

python/servo/command_base.py

@@ -302,6 +302,9 @@ class CommandBase(object):
         if hosts_file_path:
             env['HOST_FILE'] = hosts_file_path
 
+        env['RUSTDOC'] = path.join(self.context.topdir, 'etc', 'rustdoc-with-private')
+        env['RUSTC'] = path.join(self.context.topdir, 'etc', 'rustc-with-gold')
+
         return env
 
     def servo_crate(self):

python/servo/post_build_commands.py

@@ -156,10 +156,8 @@ class MachCommands(CommandBase):
                     else:
                         copy2(full_name, destination)
 
-        env = self.build_env()
-        env['RUSTDOC'] = '../../etc/rustdoc-with-private'
         return subprocess.call(["cargo", "doc"] + params,
-                               env=env, cwd=self.servo_crate())
+                               env=self.build_env(), cwd=self.servo_crate())
 
     @Command('browse-doc',
              description='Generate documentation and open it in a web browser',