Ensure current_observed_frame always points to an actually observed frame.
This solution has a caveat of being O(stack size), with the worst case occurring if there are a lot of frames between the current and previous observed frames.
An O(1) solution would require keeping track of the previous observed frame, which would require some additional frame attached metadata, which is best not attempted in an already released version.
The motivation for this change is to prevent extensions from having to check executor globals for the current execute_data during function call init. A previous implementation of the observer API initialized the function call from runtime cache initialization before execute_data was allocated which is why zend_function was passed in.
But now that the observer API is implemented via opcode specialization, it makes sense to pass in the execute_data. This also keeps the API a bit more consistent for existing extensions that already hook zend_execute_ex.
Closes GH-6209
The `zend_system_id` is a (true global) system ID that fingerprints a process state. When extensions add engine hooks during MINIT/startup, entropy is added the system ID for each hook. This allows extensions to identify that changes have been made to the engine since the last PHP process restart.
Closes GH-5871
