arm: [MVE intrinsics] add store_scatter_offset shape

This patch adds the store_scatter_offset shape and uses a new helper
class (store_scatter), which will also be used by later patches.

gcc/ChangeLog:

	* config/arm/arm-mve-builtins-shapes.cc (struct store_scatter): New.
	(struct store_scatter_offset_def): New.
	* config/arm/arm-mve-builtins-shapes.h (store_scatter_offset): New.