[clang] [clang][AVR] Improve compatibility of inline assembly with avr-gcc (PR #136534)

[Ben Shi via cfe-commits]

benshi001 wrote:

> gcc doesn't seem to allow it? https://godbolt.org/z/4zh8TTPac

avr-gcc allow value 64 for constraint 'I' in very special case, such as https://github.com/avrdudes/avr-libc/issues/678.

And my solution is loosen that check in the frontend, but let the AVR backend to deny the illegal constraint value 64.

https://github.com/llvm/llvm-project/pull/136534