[AVR] Fix incorrect calling convention for varargs functions

[benshi001]

Summary

An i8 argument should only cost 1 byte on the stack. This is
compatible with avr-gcc.

There are also more test cases (of calling convention) are added.

[benshi001]

This patch fixes https://github.com/llvm/llvm-project/issues/54148

[benshi001]

The failure information is caused by the build machines, not my patch.

[aykevl]

I assume you've verified the new test cases with avr-gcc? If so, this looks good to me.

[benshi001]

In D121767#3401166, @aykevl wrote:

I assume you've verified the new test cases with avr-gcc? If so, this looks good to me.

Yes. Verified with avr-gcc 7.3.0.