[llvm] [AVR] Support stack realignment (PR #187129)

[Ben Shi via llvm-commits]

benshi001 wrote:

@Patryk27 Do you still have the previous implementation which hold the frame pointer in an extra register pair ? I would like to see it once more. As your previous implementation, can we keep the frame pointer in `R7:R6` for avr devices, while `R19:R18` for avr tiny devices? Since they are all callee saved registers.

https://github.com/llvm/llvm-project/pull/187129