BUG: rfloordiv with fill_value, closes#27464
- [x] closes #27464, closes #26793
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Also addresses #26793, though needs test for that test added for #26793