aarch64: Re-implement vabal_high* intrinsics using builtins

aarch64: Re-implement vabal_high* intrinsics using builtins

This patch reimplements the vabal_high* intrinsics using RTL builtins.
It's straightforward, defining new unspecs and a new pattern.

gcc/ChangeLog:

	* config/aarch64/aarch64-simd-builtins.def (sabal2): Define
	builtin.
	(uabal2): Likewise.
	* config/aarch64/aarch64-simd.md (aarch64_<sur>abal2<mode>): New
	pattern.
	* config/aarch64/aarch64.md (unspec): Add UNSPEC_SABAL2 and
	UNSPEC_UABAL2.
	* config/aarch64/arm_neon.h (vabal_high_s8): Reimplement using
	builtin.
	(vabal_high_s16): Likewise.
	(vabal_high_s32): Likewise.
	(vabal_high_u8): Likewise.
	(vabal_high_u16): Likewise.
	(vabal_high_u32): Likewise.
	* config/aarch64/iterators.md (ABAL2): New mode iterator.
	(sur): Handle UNSPEC_SABAL2, UNSPEC_UABAL2.