Fix compile warning in GMP
Fix bug #65997 by switching to Serializable for GMP

Rather than using get_properties and __wakeup for serialization
the code now uses Serializable::serialize() and
Serializable::unserialize(). The get_properties handler is switched
to a get_debug_info handler. Thus get_gc will now return only
the normal properties and not do any modifications, thus fixing
the leak. This also avoids a $num property from being publicly
visible after the object was dumped or serialized, so that's an
extra plus.