aarch64: Reimplement vabdl_high* intrinsics using builtins

aarch64: Reimplement vabdl_high* intrinsics using builtins

This patch reimplements the vabdl_high intrinsics using builtins.
It slightly cleans up the RTL pattern (the mode iterators) but nothing
interesting apart from that.

gcc/ChangeLog:

	* config/aarch64/aarch64-simd-builtins.def (sabdl2, uabdl2):
	Define builtins.
	* config/aarch64/aarch64-simd.md (aarch64_<sur>abdl2<mode>_3):
	Rename to...
	(aarch64_<sur>abdl2<mode>): ... This.
	(<sur>sadv16qi): Adjust use of above.
	* config/aarch64/arm_neon.h (vabdl_high_s8): Reimplement using
	builtin.
	(vabdl_high_s16): Likewise.
	(vabdl_high_s32): Likewise.
	(vabdl_high_u8): Likewise.
	(vabdl_high_u16): Likewise.
	(vabdl_high_u32): Likewise.