Avoid BigInteger polyfill, use private methods instead

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: EgorBo

src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/Polyfills.cs

@@ -4,7 +4,6 @@
 using System.Buffers.Binary;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
-using System.Numerics;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 
@@ -20,42 +19,6 @@ internal static class DateTimeOffsetPolyfills
         }
     }
 
-    internal static class BigIntegerPolyfills
-    {
-        extension(BigInteger self)
-        {
-            public byte[] ToByteArray(bool isUnsigned, bool isBigEndian)
-            {
-                byte[] littleEndianBytes = self.ToByteArray();
-
-                if (littleEndianBytes.Length == 1)
-                    return littleEndianBytes;
-
-                Span<byte> bytesAsSpan = littleEndianBytes;
-
-                if (isBigEndian)
-                    bytesAsSpan.Reverse();
-
-                if (isUnsigned)
-                {
-                    int start = 0;
-                    for (int i = 0; i < bytesAsSpan.Length; i++)
-                    {
-                        if (bytesAsSpan[i] == 0x00)
-                            start++;
-                        else
-                            break;
-                    }
-
-                    if (start > 0)
-                        return bytesAsSpan.Slice(start).ToArray();
-                }
-
-                return littleEndianBytes;
-            }
-        }
-    }
-
     internal static class DecimalPolyfills
     {
         extension(decimal)

src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/Writer/CborWriter.Tag.cs

@@ -93,7 +93,7 @@ public void WriteBigInteger(BigInteger value)
         {
             bool isUnsigned = value.Sign >= 0;
             BigInteger unsignedValue = isUnsigned ? value : -1 - value;
-            byte[] unsignedBigEndianEncoding = unsignedValue.ToByteArray(isUnsigned: true, isBigEndian: true);
+            byte[] unsignedBigEndianEncoding = CreateUnsignedBigEndianBytes(unsignedValue);
 
             WriteTag(isUnsigned ? CborTag.UnsignedBigNum : CborTag.NegativeBigNum);
             WriteByteString(unsignedBigEndianEncoding);
@@ -207,5 +207,31 @@ public static decimal Reconstruct(decimal mantissa, long exponent)
                 }
             }
         }
+
+        private static byte[] CreateUnsignedBigEndianBytes(BigInteger value)
+        {
+#if NET
+            return value.ToByteArray(isUnsigned: true, isBigEndian: true);
+#else
+            byte[] littleEndianBytes = value.ToByteArray();
+
+            if (littleEndianBytes.Length == 1)
+                return littleEndianBytes;
+
+            Span<byte> bytesAsSpan = littleEndianBytes;
+            bytesAsSpan.Reverse();
+
+            int start = 0;
+            for (int i = 0; i < bytesAsSpan.Length; i++)
+            {
+                if (bytesAsSpan[i] == 0x00)
+                    start++;
+                else
+                    break;
+            }
+
+            return start == 0 ? littleEndianBytes : bytesAsSpan.Slice(start).ToArray();
+#endif
+        }
     }
 }