Add support for s390x (IBM Z) architecture

[Jawahars]

Add support for s390x (IBM Z) architecture

Description

The library currently doesn't recognize s390x architecture (IBM Z/zLinux), causing it to fall back to "unknown" architecture, which results in the Runtime Identifier (RID) being "linux-unknown" instead of "linux-s390x".

Current Behavior

When running on s390x architecture, RuntimeInformation.ProcessArchitecture returns a value not handled in the switch statement in GraphvizCommand.cs, causing it to hit the default case and return "unknown".

var arch = RuntimeInformation.ProcessArchitecture switch
{
    Architecture.X64 => "x64",
    Architecture.Arm64 => "arm64",
    Architecture.X86 => "x86",
    Architecture.Arm => "arm",
    _ => "unknown"  // s390x falls here
};

This results in the library looking for binaries in /runtimes/linux-unknown/native/ instead of /runtimes/linux-s390x/native/.

Expected Behavior

The library should recognize s390x architecture and use the correct RID "linux-s390x".

Environment

• Architecture: s390x (IBM Z/zLinux)
• .NET Runtime: .NET 8.0
• OS: Red Hat UBI 10 (RHEL-based)

Proposed Solution

Add s390x to the architecture switch statement:

var arch = RuntimeInformation.ProcessArchitecture switch
{
    Architecture.X64 => "x64",
    Architecture.Arm64 => "arm64",
    Architecture.X86 => "x86",
    Architecture.Arm => "arm",
    Architecture.S390x => "s390x",
    _ => "unknown"
};

Workaround

Currently, we work around this by creating symlinks from linux-unknown/native/ to the actual s390x binaries.

Additional Context

• s390x is a widely used architecture in enterprise environments (IBM Z mainframes)
• .NET has official support for s390x since .NET 6
• The Architecture.S390x enum value is available in System.Runtime.InteropServices.RuntimeInformation

I'd be happy to submit a PR with this fix if that's ok!