arm: [MVE intrinsics] add support for p8 and p16 polynomial types

arm: [MVE intrinsics] add support for p8 and p16 polynomial types

Although they look like aliases for u8 and u16, we need to define them
so that we can handle p8 and p16 suffixes with the general framework.

They will be used by vmull[bt]q_poly intrinsics.

2023-08-14  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* config/arm/arm-mve-builtins.cc (type_suffixes): Handle poly_p
	field..
	(TYPES_poly_8_16): New.
	(poly_8_16): New.
	* config/arm/arm-mve-builtins.def (p8): New type suffix.
	(p16): Likewise.
	* config/arm/arm-mve-builtins.h (enum type_class_index): Add
	TYPE_poly.
	(struct type_suffix_info): Add poly_p field.