arm: [MVE intrinsics] add load_gather_base shape

This patch adds the load_gather_base shape description.

Unlike other load_gather shapes, this one does not support overloaded
forms.

gcc/ChangeLog:

	* config/arm/arm-mve-builtins-shapes.cc (struct
	load_gather_base_def): New.
	* config/arm/arm-mve-builtins-shapes.h: (load_gather_base): New.