[AVR] Fix incorrect calling convention for varargs functions

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.

Reviewed By: aykevl, dylanmckay

Differential Revision: https://reviews.llvm.org/D121767

(cherry picked from commit 3fd9a320)