Use AddAcross for reduce sum, if available

Author: brianpopow

src/ImageSharp/Common/Helpers/Numerics.cs

@@ -817,6 +817,12 @@ public static int ReduceSum(Vector256<int> accumulator)
     [MethodImpl(InliningOptions.ShortMethod)]
     public static int ReduceSumArm(Vector128<uint> accumulator)
     {
+        if (AdvSimd.Arm64.IsSupported)
+        {
+            Vector64<uint> sum = AdvSimd.Arm64.AddAcross(accumulator);
+            return (int)AdvSimd.Extract(sum, 0);
+        }
+
         Vector128<ulong> sum2 = AdvSimd.AddPairwiseWidening(accumulator);
         Vector64<uint> sum3 = AdvSimd.Add(sum2.GetLower().AsUInt32(), sum2.GetUpper().AsUInt32());
         return (int)AdvSimd.Extract(sum3, 0);