[avr] Return values are promoted to (unsigned) int for no reason.

Compile with `--target=avr -mmcu=atmega8 -Os -save-temps` the following code:
```c++
char func (char c)
{
    return c;
}
```
The generated assembly reads:
```asm
func:
	mov	r25, r24
	lsl	r25
	sbc	r25, r25
	ret
```
which just wastes 3 cycles and 6 bytes for integer promotion that's not required.