Add observer temporary to dl'ed functions

[arnaud-lb]

Possible fix for GH-17211.

See #17211 (comment).

When observer is enabled, we normally add an extra temporary to all functions, to store the previously observed frame. However, this is done in zend_observer_post_startup() so it doesn't happen to dl'ed() functions.

One possible fix would be to move that from zend_observer_post_startup() to zend_register_functions(), but this would be too early: Observer may not be enabled when zend_register_functions() is called, and may still be enabled later.

However, when zend_register_functions() is called at run-time (during dl()), we know definitively whether observer is enabled.

Here I update zend_register_functions() to add a temporary to dl'ed() functions when observer is enabled.