GH-114847: Speed up `posixpath.realpath()` (#114848)

Apply the following optimizations to `posixpath.realpath()`:

- Remove use of recursion
- Construct child paths directly rather than using `join()`
- Use `os.getcwd[b]()` rather than `abspath()`
- Use `startswith(sep)` rather than `isabs()`
- Use slicing rather than `split()`

Co-authored-by: Petr Viktorin <encukou@gmail.com>