Revert "Don't pin `val` passed in to `rb_define_const`."

I think this is breaking something

This reverts commit 1be84e53d7.

variable.c

@@ -3154,6 +3154,7 @@ rb_define_const(VALUE klass, const char *name, VALUE val)
     if (!rb_is_const_id(id)) {
 	rb_warn("rb_define_const: invalid name `%s' for constant", name);
     }
+    rb_gc_register_mark_object(val);
     rb_const_set(klass, id, val);
 }