Add support for s390x (IBM Z) architecture (#119)

Add support for s390x (IBM Z) architecture

Author: Jawahars

Rubjerg.Graphviz/GraphvizCommand.cs

@@ -27,6 +27,9 @@ internal static string Rid
                 Architecture.Arm64 => "arm64",
                 Architecture.X86 => "x86",
                 Architecture.Arm => "arm",
+                // Cast allows compilation in .NET Standard 2.0/2.1.
+                // (Architecture)5 is S390x, added in .NET 6.
+                (Architecture)5 => "s390x",
                 _ => "unknown"
             };