Revert "Remove gold hack, since it's in rustc automatically now"

This reverts commit 2955d89.

etc/rustc-with-gold

@@ -0,0 +1,2 @@
+#!/bin/sh
+rustc -C link-args=-fuse-ld=gold "$@"

python/servo/command_base.py

@@ -11,6 +11,7 @@
 from os import path
 import contextlib
 import subprocess
+from subprocess import PIPE
 import sys
 import toml
 
@@ -153,6 +154,7 @@ def resolverelative(category, key):
         if not self.config["tools"]["system-cargo"]:
             self.config["tools"]["cargo-root"] = path.join(
                 context.sharedir, "cargo", self.cargo_build_id())
+        self.config["tools"].setdefault("rustc-with-gold", True)
 
         self.config.setdefault("build", {})
         self.config["build"].setdefault("android", False)
@@ -377,6 +379,10 @@ def build_env(self, gonk=False, hosts_file_path=None):
 
         env['RUSTDOC'] = path.join(self.context.topdir, 'etc', 'rustdoc-with-private')
 
+        if self.config["tools"]["rustc-with-gold"]:
+            if subprocess.call(['which', 'ld.gold'], stdout=PIPE, stderr=PIPE) == 0:
+                env['RUSTC'] = path.join(self.context.topdir, 'etc', 'rustc-with-gold')
+
         return env
 
     def servo_crate(self):