[avr] Convert the backend to MODE_CC so it can be kept in future releases

This is a tracker bug for the convert the avr backend from CC0 to MODE_CC so it can be kept in future releases. See: https://gcc.gnu.org/wiki/CC0Transition I will be using this bug report to create a bounty on BountySource.com. We have already successfully funded such a conversion for the m68k backend, see #91851.