[gn] Make v8_target_arch a GN declare_arg()

There are bots on the chromium.lkgr waterfall that need to
be able to set v8_target_arch to "arm" while the rest of
the target build is "x86". This CL makes that argument
a declare_arg(), so that we can do that.

BUG=chromium:605732,chromium:474921
NOTRY=true

Review-Url: https://codereview.chromium.org/2007843002
Cr-Commit-Position: refs/heads/master@{#36464}

BUILD.gn

@@ -11,8 +11,7 @@ if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
-# Because standalone V8 builds are not supported, assume this is part of a
-# Chromium build.
+import("gni/v8.gni")
 import("//build_overrides/v8.gni")
 
 import("snapshot_toolchain.gni")
@@ -70,13 +69,15 @@ declare_args() {
 v8_random_seed = "314159265"
 v8_toolset_for_shell = "host"
 
-if (is_msan) {
+if (v8_target_arch == "") {
+  if (is_msan) {
     # Running the V8-generated code on an ARM simulator is a powerful hack that
     # allows the tool to see the memory accesses from JITted code. Without this
     # flag, JS code causes false positive reports from MSan.
     v8_target_arch = "arm64"
-} else {
+  } else {
     v8_target_arch = target_cpu
+  }
 }
 
 if (v8_use_snapshot && v8_use_external_startup_data) {

build_overrides/v8.gni

@@ -4,16 +4,12 @@
 
 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") &&

gni/v8.gni

@@ -0,0 +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.
+
+declare_args() {
+  # V8 generates code for this architecture. If v8_target_arch differs from
+  # target_cpu, a simulator will be run.
+  v8_target_arch = ""
+}