AVR: target/107957 - Split multi-byte loads and stores.

AVR: target/107957 - Split multi-byte loads and stores.

This patch splits multi-byte loads and stores into single-byte
ones provided:

-  New option -msplit-ldst is on (e.g. -O2 and higher), and
-  The memory is non-volatile, and
-  The address space is generic, and
-  The split addresses are natively supported by the hardware.

gcc/
	PR target/107957
	* config/avr/avr.opt (-msplit-ldst, avropt_split_ldst):
	New option and associated var.
	* common/config/avr/avr-common.cc (avr_option_optimization_table)
	[OPT_LEVELS_2_PLUS]: Turn on -msplit_ldst.
	* config/avr/avr-passes.cc (splittable_address_p)
	(avr_byte_maybe_mem, avr_split_ldst): New functions.
	* config/avr/avr-protos.h (avr_split_ldst): New proto.
	* config/avr/avr.md (define_split) [avropt_split_ldst]: Run
	avr_split_ldst().