[simulator] Make reference redirection thread-safe.

src/s390/simulator-s390.cc

Index: src/s390/simulator-s390.cc
diff --git a/src/s390/simulator-s390.cc b/src/s390/simulator-s390.cc
index 896b3d5009e95f181f2c5ed6c66ed9ff3d6ccc94..9f41bace2a22dad950c03ec37cf09b205be134f8 100644
--- a/src/s390/simulator-s390.cc
+++ b/src/s390/simulator-s390.cc
@@ -1644,6 +1644,8 @@ void Simulator::TearDown(base::CustomMatcherHashMap* i_cache,
 void* Simulator::RedirectExternalReference(Isolate* isolate,
                                            void* external_function,
                                            ExternalReference::Type type) {
+  base::LockGuard<base::Mutex> lock_guard(
+      isolate->simulator_redirection_mutex());
   Redirection* redirection = Redirection::Get(isolate, external_function, type);
   return redirection->address();
 }