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

components/nacl/BUILD.gn

 # Copyright (c) 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/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 
 if (enable_nacl) {
@@ skipping 26 matching lines @@
       "//native_client/src/trusted/service_runtime:sel_main_chrome",
       "//ppapi/c",
       "//ppapi/proxy:ipc",
       "//ppapi/shared_impl",
       "//third_party/mojo/src/mojo/edk/embedder",
     ]
 
     if (enable_nacl_untrusted) {
       data_deps = [
         # TODO(GYP): handle other architectures correctly.
-        "//ppapi/native_client:nacl_irt(//native_client/build/toolchain/nacl:irt_x64)",
+        "//ppapi/native_client:nacl_irt(//build/toolchain/nacl:irt_x64)",
       ]
     }
     if (enable_pnacl) {
       data_deps += [
         # TODO(GYP): handle other architectures correctly.
         "//ppapi/native_client/src/untrusted/pnacl_support_extension",
       ]
     }
   }
 
@@ skipping 256 matching lines @@
 
   if (is_linux) {
     sources += [
       "common/nacl_paths.cc",
       "common/nacl_paths.h",
     ]
 
     defines = [ "__STDC_LIMIT_MACROS=1" ]
   }
 }