Update GN build to use v8_target_cpu instead of v8_target_arch.

gni/v8.gni

Index: gni/v8.gni
diff --git a/gni/v8.gni b/gni/v8.gni
index ec943a8fd435263281ce0772db9a63e80c10f82b..ae26521a6a98fadad77aadc422689fff9cd27141 100644
--- a/gni/v8.gni
+++ b/gni/v8.gni
@@ -2,7 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//build/config/sanitizers/sanitizers.gni")
+import("//build/config/v8_target_cpu.gni")
 
 declare_args() {
   # Turns on compiler optimizations in V8 in Debug build.
@@ -15,21 +15,6 @@ declare_args() {
   # Use external files for startup data blobs:
   # the JS builtins sources and the start snapshot.
   v8_use_external_startup_data = !is_ios
-
-  # V8 generates code for this architecture. If v8_target_arch differs from
-  # target_cpu, a simulator will be run.
-  v8_target_arch = ""
-}
-
-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 {
-    v8_target_arch = target_cpu
-  }
 }
 
 ###############################################################################