[llvm] [AVR] Support stack realignment (PR #187129)

[Ben Shi via llvm-commits]

benshi001 wrote:

It would be better to add 
```
void g(void *, void *);

void f() {
        int a = 22;
        _Alignas(8) int d = 55;
        g(&a, &d);
}
``` 
to the unit test.

https://github.com/llvm/llvm-project/pull/187129