Cleanup SPL autoload implementation

Cleanup SPL autoload implementation

Replace EG(autoload_func) with a C level zend_autoload hook.
This avoids having to do one indirection through PHP function
calls. The need for EG(autoload_func) was a leftover from the
__autoload() implementation.

Additionally, drop special-casing of spl_autoload(), and instead
register it just like any other autoloading function. This fixes
bug #71236 as a side-effect.

Finally, change spl_autoload_functions() to always return an array.
The distinction between false and an empty array no longer makes
sense here.

Closes GH-5696.