[gn] Make v8_target_arch a GN declare_arg()

gni/v8.gni

Index: gni/v8.gni
diff --git a/test/cctest/cctest_exe.isolate b/gni/v8.gni
similarity index 51%
copy from test/cctest/cctest_exe.isolate
copy to gni/v8.gni
index da5327846e7dfc1f6a7b155b984c58614de4c238..81fcfbfa7b0e3d5cf1dc3a2a9da4b7b795bd16c8 100644
--- a/test/cctest/cctest_exe.isolate
+++ b/gni/v8.gni
@@ -1,13 +1,9 @@
 # Copyright 2016 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.
-{
-  'variables': {
-    'files': [
-      '<(PRODUCT_DIR)/cctest<(EXECUTABLE_SUFFIX)',
-    ],
-  },
-  'includes': [
-    '../../src/base.isolate',
-  ],
+
+declare_args() {
+  # V8 generates code for this architecture. If v8_target_arch differs from
+  # target_cpu, a simulator will be run.
+  v8_target_arch = ""
 }