Update NaCl GN toolchain definitions.

src/trusted/service_runtime/linux/toolchain/BUILD.gn

 # Copyright (c) 2015 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/toolchain/gcc_toolchain.gni")
 
 # This toolchain's sole purpose is to compile nacl_helper_bootstrap.c.
 # That requires a separate toolchain because that's the most reasonable way
 # to instantiate the standard configs in a context where use_gold=false
 # and use_debug_fission=false when the main build is using either
 # use_gold=true or use_debug_fission=true. See ../BUILD.gn for details.
 
 template("nacl_bootstrap_toolchain") {
   clang_toolchain(target_name) {
-    toolchain_cpu = invoker.toolchain_cpu
-    toolchain_os = target_os
-    use_debug_fission = false
-    use_gold = false
+    toolchain_args = {
+      current_cpu = invoker.toolchain_cpu
+      current_os = target_os
+      use_debug_fission = false
+      use_gold = false
+    }
   }
 }
 
 nacl_bootstrap_toolchain("nacl_bootstrap_x64") {
   toolchain_cpu = "x64"
 }
 
 nacl_bootstrap_toolchain("nacl_bootstrap_x86") {
   toolchain_cpu = "x86"
 }
 
 nacl_bootstrap_toolchain("nacl_bootstrap_arm") {
   toolchain_cpu = "arm"
 }