[gn] Fix setting v8_target_arch default

gni/v8.gni

Index: gni/v8.gni
diff --git a/gni/v8.gni b/gni/v8.gni
index 81fcfbfa7b0e3d5cf1dc3a2a9da4b7b795bd16c8..6a24219c34ea5d8bd449308840ee5af9d40ad421 100644
--- a/gni/v8.gni
+++ b/gni/v8.gni
@@ -2,8 +2,21 @@
 # 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")
+
 declare_args() {
   # 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
+  }
+}