Add support for s390x (IBM Z) architecture

[Jawahars]

Add support for s390x (IBM Z) architecture

Summary

This PR adds support for s390x architecture (IBM Z/zLinux) to the Graphviz.NetWrapper library.

Problem

Currently, when running on s390x architecture, the library doesn't recognize it and falls back to "unknown", resulting in incorrect runtime identifier paths (linux-unknown instead of linux-s390x).

Solution

Added Architecture.S390x case to the architecture detection switch statement in GraphvizCommand.cs.

Changes

• Modified Rubjerg.Graphviz/GraphvizCommand.cs to include s390x architecture detection

Compatibility

• No breaking changes
• Backward compatible with existing architectures
• Uses standard .NET Architecture.S390x enum value (available since .NET 6)

Additional Context

• This enables the library to work properly on IBM Z systems without workarounds

Fixes #118

[chtenb]

Can you please make sure to test this change, and explain how you tested it, before I merge and release a new version?
The generated package can be downloaded from the CI, or you can build it yourself.

[Jawahars]

Test Environment

• Platform: IBM Z Linux (s390x architecture)
• OS: Red Hat UBI 10.1 (ubi-minimal)
• Runtime: .NET 8.0 (ASP.NET Core)
• Package Tested: Rubjerg.Graphviz 3.0.4-dev-20260309141834
• Application: Production ASP.NET Core application with graph rendering features

How I Tested

1. Since the NuGet package doesn't include pre-built native binaries for s390x, I compiled libGraphvizWrapper.so from source using gcc on s390x hardware.
2. Updated project to use the dev NuGet package via local source
3. Deployed the wrapper to runtimes/linux-s390x/native/ directory
4. Ran application in Docker container on s390x hardware and the charts are rendered using Graphviz

Key Improvement

Before this PR: The package would look for binaries in linux-unknown/native/ on s390x systems, requiring workarounds with symlinks.

After this PR: The package now correctly recognizes s390x architecture and searches linux-s390x/native/ directly, eliminating the need for linux-unknown fallback.

[chtenb]

Since our automated tests only run against .NET 8.0, I've verified manually that this causes no runtime issues in .NET versions prior to version 6.0.

[Jawahars]

Thank you @chtenb