[clang] [clang][AVR] Add AVR-specific builtin functions (PR #203214)

[via cfe-commits]

beakthoven wrote:

Taking @benshi001 comments into consideration, I think its better if we split the builtin implementation into multiple PRs. Since there is no issue regarding the implementation of the simpiler builtins (nop/sei/cli/sleep/wdr/swap) I think they can be fast tracked.

I've commenced my work on fixing the reported oversights in a separate [branch](https://github.com/beakthoven/llvm-project/commits/avr-builtins-fmul/) which will be later pushed for PR once peer review and testing can be done.

https://github.com/llvm/llvm-project/pull/203214