[php-src] Issue #22779: mb_strrpos() returns the wrong position for a negative offset in a non-UTF-8 encoding

Issue: https://github.com/php/php-src/issues/22779
Author: eyupcanakman

### Description

The following code:

```php
<?php
var_dump(mb_strrpos("\xA9\xA9X", "\xA9", -2, "ISO-8859-1"));
var_dump(strrpos("\xA9\xA9X", "\xA9", -2));
```

Resulted in this output:

```
int(0)
int(1)
```

But I expected this output instead:

```
int(1)
int(1)
```

For a single-byte encoding one byte is one character, so `mb_strrpos()` should agree with `strrpos()` here. The result is wrong for a negative offset in any non-UTF-8 encoding. An ISO-8859-1 needle byte in 0x80-0xBF gives a position that is too early, and an ASCII needle in UTF-16 also returns the wrong position.

### PHP Version

PHP 8.4.25-dev (the same code is in 8.3, 8.5 and master)

### Operating System

macOS 27.0