aarch64: Use type-qualified builtins for USUB[LW][2] Neon intrinsics

aarch64: Use type-qualified builtins for USUB[LW][2] Neon intrinsics

Declare unsigned type-qualified builtins and use them to implement
widening-subtract Neon intrinsics. This removes the need for many
casts in arm_neon.h.

gcc/ChangeLog:

2021-11-09  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64-simd-builtins.def: Use BINOPU type
	qualifiers in generator macros for usub[lw][2] builtins.
	* config/aarch64/arm_neon.h (vsubl_s8): Remove unnecessary
	cast.
	(vsubl_s16): Likewise.
	(vsubl_s32): Likewise.
	(vsubl_u8): Use type-qualified builtin and remove casts.
	(vsubl_u16): Likewise.
	(vsubl_u32): Likewise.
	(vsubl_high_s8): Remove unnecessary cast.
	(vsubl_high_s16): Likewise.
	(vsubl_high_s32): Likewise.
	(vsubl_high_u8): Use type-qualified builtin and remove casts.
	(vsubl_high_u16): Likewise.
	(vsubl_high_u32): Likewise.
	(vsubw_s8): Remove unnecessary casts.
	(vsubw_s16): Likewise.
	(vsubw_s32): Likewise.
	(vsubw_u8): Use type-qualified builtin and remove casts.
	(vsubw_u16): Likewise.
	(vsubw_u32): Likewise.
	(vsubw_high_s8): Remove unnecessary cast.
	(vsubw_high_s16): Likewise.
	(vsubw_high_s32): Likewise.
	(vsubw_high_u8): Use type-qualified builtin and remove casts.
	(vsubw_high_u16): Likewise.
	(vsubw_high_u32): Likewise.