add nacl_toolchain to //build and flip to using it in the GN build.

ppapi/native_client/BUILD.gn

 # Copyright 2015 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.
 
 import("//build/config/features.gni")
 
 if (is_nacl) {
   group("ppapi_lib") {
     deps = [
       "//native_client/src/untrusted/pthread",
@@ skipping 42 matching lines @@
       "${root_build_dir}/{{source_name_part}}_${suffix}.nexe",
     ]
     deps = [
       ":nacl_irt",
     ]
   }
 }
 
 group("irt") {
   deps = [
-    ":nacl_irt_copy(//native_client/build/toolchain/nacl:irt_${target_cpu})",
+    ":nacl_irt_copy(//build/toolchain/nacl:irt_${target_cpu})",
   ]
 }