Add -Xzygote to suppress starting the signal catcher thread.

Change-Id: Iae98ac9d362c71074befbc702f8cae96888211ec
diff --git a/src/runtime.h b/src/runtime.h
index 65a54e9..462bb81 100644
--- a/src/runtime.h
+++ b/src/runtime.h
@@ -51,6 +51,7 @@
     std::vector<std::string> images_;
     bool check_jni_;
     std::string jni_trace_;
+    bool is_zygote_;
     size_t heap_initial_size_;
     size_t heap_maximum_size_;
     size_t stack_size_;
@@ -208,6 +209,7 @@
   void StartDaemonThreads();
 
   bool verbose_startup_;
+  bool is_zygote_;
 
   // The host prefix is used during cross compilation. It is removed
   // from the start of host paths such as: