Don't save registers in main().

Don't save registers in main().

gcc/
	Don't save registers in main().
	PR target/83737
	* doc/invoke.texi (AVR Options) [-mmain-is-OS_task]: Document it.
	* config/avr/avr.opt (-mmain-is-OS_task): New target option.
	* config/avr/avr.c (avr_set_current_function): Don't error if
	naked, OS_task or OS_main are specified at the same time.
	(avr_function_ok_for_sibcall): Don't disable sibcalls for OS_task,
	OS_main.
	(avr_insert_attributes) [-mmain-is-OS_task] <main>: Add OS_task
	attribute.
	* common/config/avr/avr-common.c (avr_option_optimization_table):
	Switch on -mmain-is-OS_task for optimizing compilations.

From-SVN: r256373