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

ppapi/native_client/src/untrusted/pnacl_support_extension/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 (enable_nacl && enable_nacl_untrusted && enable_pnacl) {
   # We could use 'copies', but we want to rename the files
   # in a white-listed way first.  Thus use an action.
   action("pnacl_support_extension") {
     script = "pnacl_component_crx_gen.py"
     inputs = [
       "//native_client/build/package_version/package_version.py",
       "//native_client/pnacl/driver/pnacl_info_template.json",
       "//native_client/toolchain_revisions/pnacl_translator.json",
     ]
     shim_target_dir = "//ppapi/native_client/src/untrusted/pnacl_irt_shim/"
     shim_target_out_dir =
         rebase_path(get_path_info(shim_target_dir, "out_dir"), root_build_dir)
     src_shim_name = "libpnacl_irt_shim_browser.a"
     desired_shim_name = "libpnacl_irt_shim.a"
     shim_target_label = "$shim_target_dir:browser"
 
     # Use the IRT toolchain to build the shim. We want similar properties
     # (x86-64 sandbox base address hiding). One thing we don't want is the
     # IRT's secondary TLS, but that is handled by tls_edit and not compiler
     # flags (so IRT compiler flags should be fine).
-    shim_toolchain_base = "//native_client/build/toolchain/nacl:irt_"
+    shim_toolchain_base = "//build/toolchain/nacl:irt_"
+
     shim_target_tc_label = "$shim_target_label($shim_toolchain_base$target_cpu)"
     deps = [
       shim_target_tc_label,
     ]
 
     # Figure out the path of the shim library that we depend upon.
     # TODO(jvoung): change this to:
     #   lib_path = rebase_path(
     #      get_label_info(shim_target_tc_label, "target_out_dir"),
     #      root_build_dir)
@@ skipping 94 matching lines @@
       rebase_path("//native_client/build/package_version/package_version.py",
                   root_build_dir),
       "--pnacl_package_name",
       "pnacl_translator",
 
       # ABI Version Number
       "1",
     ]
   }
 }