opcache: Call orig_post_shutdown_cb()

This was never called and resulted in this compiler warning:

```
./ext/opcache/ZendAccelerator.c:3720:15: warning: variable 'orig_post_shutdown_cb' set
      but not used [-Wunused-but-set-global]
 3720 | static void (*orig_post_shutdown_cb)(void);
      |               ^
```

Closes GH-24761.

Author: ndossche

NEWS

@@ -157,6 +157,7 @@ PHP                                                                        NEWS
     handled at the end of a loop trace. (Levi Morrison)
   . Fixed bug GH-24675 (function with >~1.7 MB of machine code corrupts JIT
     buffer accounting). (ndossche)
+  . Call orig_post_shutdown_cb(). (ndossche)
 
 - PDO:
   . Fixed invalid ATTR_ORACLE_NULLS values being accepted and truncated.

ext/opcache/ZendAccelerator.c

@@ -3429,6 +3429,11 @@ static void (*orig_post_shutdown_cb)(void);
 
 static void accel_post_shutdown(void)
 {
+	/* Call original shutdown prior to detaching SHM, so it can access its memory safely. */
+	if (orig_post_shutdown_cb) {
+		orig_post_shutdown_cb();
+	}
+
 	zend_shared_alloc_shutdown();
 }