GN: make current_cpu work

gn/BUILDCONFIG.gn

 # Copyright 2016 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # It's best to keep the names and defaults of is_foo flags consistent with Chrome.
 
 declare_args() {
   is_debug = true
   is_component_build = false
 }
 
 # Platform detection
 if (target_os == "") {
   target_os = host_os
 }
 if (current_os == "") {
   current_os = target_os
 }
 
+if (target_cpu == "") {
+  target_cpu = host_cpu
+}
+if (current_cpu == "") {
+  current_cpu = target_cpu
+}
+
 is_android = current_os == "android"
 is_fuchsia = current_os == "fuchsia"
 is_ios = current_os == "ios"
 is_linux = current_os == "linux"
 is_mac = current_os == "mac"
 is_win = current_os == "win"
 
 is_posix = !is_win
 
 # A component is either a static or a shared library.
@@ skipping 32 matching lines @@
 set_defaults("shared_library") {
   configs = default_configs
 }
 
 set_defaults("component") {
   configs = default_configs
 }
 
 # For now, we support GCC-like toolchains, including Clang.
 set_default_toolchain("//gn:gcc_like")