[llvm] [AVR] Support return address intrinsics (PR #67266)

[Ben Shi via llvm-commits]

benshi001 wrote:

I suggest you split your patch to two ones. 

The first one fixes the wrong operation SPREAD/SPWRITE, at least this part I think it is good. 

Regarding the second part, I think there are still some spots, especially you need to distinguish devices with 2-byte PC and 3-byte PC, which does affect the address of return address on the stack.

https://github.com/llvm/llvm-project/pull/67266