__WORKERD_CX__ define to identify when workerd-cxx fork is in use

Author: mikea

include/cxx.h

@@ -20,6 +20,10 @@
 #include <sys/types.h>
 #endif
 
+#ifndef __WORKERD_CXX__
+#define __WORKERD_CXX__
+#endif
+
 namespace rust {
 inline namespace cxxbridge1 {