AVR: Use types of exact size and signedness in built-ins.

AVR: Use types of exact size and signedness in built-ins.

The AVR built-ins used types like "int" or "char" that don't
have exact signedness or type size which depend on -mint8
and -f[no-][un-]signed-char etc.  As the built-ins are modelling
machine instructions of given type sizes and signedness, also
use according types in their prototypes.

gcc/
	* config/avr/builtins.def: Use function prototypes of given size
	and signedness.
	* config/avr/avr.cc (avr_init_builtins): Adjust types required
	by builtins.def.
	* doc/extend.texi (AVR Built-in Functions): Adjust accordingly.