aarch64: Use type-qualified builtins for U[R]HADD Neon intrinsics

aarch64: Use type-qualified builtins for U[R]HADD Neon intrinsics

Declare unsigned type-qualified builtins and use them to implement
(rounding) halving-add 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 u[r]hadd builtins.
	* config/aarch64/arm_neon.h (vhadd_s8): Remove unnecessary
	cast.
	(vhadd_s16): Likewise.
	(vhadd_s32): Likewise.
	(vhadd_u8): Use type-qualified builtin and remove casts.
	(vhadd_u16): Likewise.
	(vhadd_u32): Likewise.
	(vhaddq_s8): Remove unnecessary cast.
	(vhaddq_s16): Likewise.
	(vhaddq_s32): Likewise.
	(vhaddq_u8): Use type-qualified builtin and remove casts.
	(vhaddq_u16): Likewise.
	(vhaddq_u32): Likewise.
	(vrhadd_s8): Remove unnecessary cast.
	(vrhadd_s16): Likewise.
	(vrhadd_s32): Likewise.
	(vrhadd_u8): Use type-qualified builtin and remove casts.
	(vrhadd_u16): Likewise.
	(vrhadd_u32): Likewise.
	(vrhaddq_s8): Remove unnecessary cast.
	(vrhaddq_s16): Likewise.
	(vrhaddq_s32): Likewise.
	(vrhaddq_u8): Use type-wualified builtin and remove casts.
	(vrhaddq_u16): Likewise.
	(vrhaddq_u32): Likewise.