GN: Propagate top-level host_toolchain through to all toolchains

build/toolchain/mac/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # TODO(brettw) Use "gcc_toolchain.gni" like the Linux toolchains. This requires
 # some enhancements since the commands on Mac are slightly different than on
 # Linux.
 
 import("../goma.gni")
 import("//build/config/ios/ios_sdk.gni")
@@ skipping 219 matching lines @@
     tool("copy") {
       command = copy_command
       description = copy_description
     }
 
     toolchain_args() {
       current_cpu = invoker.toolchain_cpu
       current_os = invoker.toolchain_os
 
       # These values need to be passed through unchanged.
+      host_toolchain = host_toolchain
       target_os = target_os
       target_cpu = target_cpu
 
       if (defined(invoker.is_clang)) {
         is_clang = invoker.is_clang
       }
     }
   }
 }
 
@@ skipping 44 matching lines @@
 }
 
 mac_toolchain("x64") {
   toolchain_cpu = "x64"
   toolchain_os = "mac"
   cc = "${goma_prefix}/gcc"
   cxx = "${goma_prefix}/g++"
   ld = cxx
   is_clang = false
 }