Mono doesn't have BCrypt.dll

Author: scott-xu

src/Renci.SshNet/Security/Cryptography/Ciphers/AesGcmCipher.cs

@@ -65,7 +65,15 @@ public AesGcmCipher(byte[] key, byte[] iv, int aadLength)
 #if !NETSTANDARD
             if (System.Security.Cryptography.AesGcm.IsSupported)
             {
-                _impl = new BclImpl(key, _iv);
+                try
+                {
+                    _impl = new BclImpl(key, _iv);
+                }
+                catch (DllNotFoundException)
+                {
+                    // Mono doesn't have BCrypt.dll
+                    _impl = new BouncyCastleImpl(key, _iv);
+                }
             }
             else
 #endif