Make byte order for X509Certificate2::GetSerialNumber on Unix match Windows.

PropsTests::TestSerial caught that the byte ordering for GetSerialNumber was big-endian on Unix systems, and little-endian on Windows.

Author: bartonjs

src/System.Security.Cryptography.X509Certificates/src/Internal/Cryptography/Pal.Unix/OpenSslX509CertificateReader.cs

@@ -140,7 +140,12 @@ public byte[] SerialNumber
             get
             {
                 IntPtr serialNumberPtr = Interop.libcrypto.X509_get_serialNumber(_cert);
-                return Interop.NativeCrypto.GetAsn1StringBytes(serialNumberPtr);
+                byte[] serial = Interop.NativeCrypto.GetAsn1StringBytes(serialNumberPtr);
+
+                // Windows returns this in BigInteger Little-Endian,
+                // OpenSSL returns this in BigInteger Big-Endian.
+                Array.Reverse(serial);
+                return serial;
             }
         }