Fix XxHash3 DeriveSecretFromSeed scalar fallback on big endian (#78084)

Fixes #78043

Author: stephentoub

src/libraries/System.IO.Hashing/src/System/IO/Hashing/XxHash3.cs

@@ -713,8 +713,8 @@ private static void DeriveSecretFromSeed(byte* destinationSecret, ulong seed)
                 {
                     for (int i = 0; i < SecretLengthBytes; i += sizeof(ulong) * 2)
                     {
-                        Unsafe.WriteUnaligned(destinationSecret + i, Unsafe.ReadUnaligned<ulong>(defaultSecret + i) + seed);
-                        Unsafe.WriteUnaligned(destinationSecret + i + 8, Unsafe.ReadUnaligned<ulong>(defaultSecret + i + 8) - seed);
+                        WriteUInt64LE(destinationSecret + i, ReadUInt64LE(defaultSecret + i) + seed);
+                        WriteUInt64LE(destinationSecret + i + 8, ReadUInt64LE(defaultSecret + i + 8) - seed);
                     }
                 }
             }
@@ -1003,6 +1003,16 @@ private static ulong ReadUInt64LE(byte* data) =>
                 Unsafe.ReadUnaligned<ulong>(data) :
                 BinaryPrimitives.ReverseEndianness(Unsafe.ReadUnaligned<ulong>(data));
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static void WriteUInt64LE(byte* data, ulong value)
+        {
+            if (!BitConverter.IsLittleEndian)
+            {
+                value = BinaryPrimitives.ReverseEndianness(value);
+            }
+            Unsafe.WriteUnaligned(data, value);
+        }
+
         [StructLayout(LayoutKind.Auto)]
         private struct State
         {