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

third_party/mojo/src/nacl_bindings/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.
 
 # Trusted code
 if (!is_nacl) {
   # A library for launching a NaCl sandbox connected to a Mojo embedder.
   source_set("monacl_sel") {
     sources = [
+      "mojo_syscall.cc",
       "mojo_syscall_internal.h",
-      "mojo_syscall.cc",
       "monacl_sel_main.cc",
     ]
     deps = [
       "../mojo/public/c/system",
 
       # This target makes sure we have all the pre-processor defines needed to
       # use NaCl's headers.
-      "//native_client/build/config/nacl:nacl_base",
+      "//build/config/nacl:nacl_base",
       "//native_client/src/trusted/desc:nrd_xfer",
       "//native_client/src/trusted/service_runtime:sel_main_chrome",
     ]
   }
 }
 
 # Untrusted code
 if (is_nacl) {
   executable("irt_mojo") {
     cflags_c = [ "-std=c99" ]
     sources = [
       "../mojo/public/platform/nacl/mojo_irt.h",
       "irt_entry_mojo.c",
       "mojo_irt.c",
     ]
     deps = [
       "../mojo/public/c/system",
-      "//native_client/build/config/nacl:nacl_base",
+      "//build/config/nacl:nacl_base",
       "//native_client/src/untrusted/irt:irt_core_lib",
       "//native_client/src/untrusted/nacl:imc_syscalls",
     ]
   }
 }