Introduce --zap-cpp-pointers (off by default)

src/execution.h

Index: src/execution.h
diff --git a/src/execution.h b/src/execution.h
index d5f6371726f1f8a837d419773a398827bfba4869..a889f4682f4c475c76921a81b04b7a3937fb5cfc 100644
--- a/src/execution.h
+++ b/src/execution.h
@@ -135,6 +135,10 @@ class V8_EXPORT_PRIVATE StackGuard final {
   Object* HandleInterrupts();
   void HandleGCInterrupt();
 
+#if DEBUG
+  uintptr_t stack_base_position() { return thread_local_.stack_base_position_; }
+#endif
+
  private:
   StackGuard();
 
@@ -214,6 +218,9 @@ class V8_EXPORT_PRIVATE StackGuard final {
 
     PostponeInterruptsScope* postpone_interrupts_;
     int interrupt_flags_;
+#if DEBUG
+    uintptr_t stack_base_position_;
+#endif
   };
 
   // TODO(isolates): Technically this could be calculated directly from a