[gn] Make v8_target_arch a GN declare_arg()

build_overrides/v8.gni

 # Copyright 2015 The V8 project 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")
 import("//build/config/ui.gni")
+import("//gni/v8.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 
-declare_args() {
-  # V8 generates code for this architecture.
-  v8_target_arch = target_cpu
-}
-
 if (((v8_target_arch == "ia32" ||
       v8_target_arch == "x64" ||
       v8_target_arch=="x87") &&
      (is_linux || is_mac)) ||
     (v8_target_arch == "ppc64" && is_linux)) {
   v8_enable_gdbjit = true
 }
 
 icu_use_data_file = false
 v8_imminent_deprecation_warnings = true
 v8_optimized_debug = false
 v8_extra_library_files = []
 v8_experimental_extra_library_files = []