Properly set the message of Exceptions on TruffleRuby

* From truffleruby/truffleruby@1f81db8

Author: eregon

ext/psych/psych_to_ruby.c

@@ -10,7 +10,11 @@ static VALUE build_exception(VALUE self, VALUE klass, VALUE mesg)
 {
     VALUE e = rb_obj_alloc(klass);
 
+#ifdef TRUFFLERUBY
+    rb_exc_set_message(e, mesg);
+#else
     rb_iv_set(e, "mesg", mesg);
+#endif
 
     return e;
 }