Removed mrb_jmpbuf_id for C++ exceptions

Use stack variable addresses as identifiers instead of global variable values.
Since the stack variable address is uniquely determined within the call, there is no need to maintain a global variable.

src/vm.c

@@ -3120,7 +3120,3 @@ mrb_top_run(mrb_state *mrb, const struct RProc *proc, mrb_value self, mrb_int st
   }
   return mrb_vm_run(mrb, proc, self, stack_keep);
 }
-
-#if defined(MRB_USE_CXX_EXCEPTION) && defined(__cplusplus)
-mrb_int mrb_jmpbuf_id = 0;
-#endif