Enable building with the Swift Linux SDK

[t089]

This adds support for compiling against the Swift Linux SDK.

Motivation and Context

Building portable mcp servers for linux is great.

How Has This Been Tested?

Used this patch to build, compile and deploy a stdio mcp server on linux.

Breaking Changes

No.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

I think with this change we could remove the !os(Linux) in Package.swift which anyhow does make much sense in the scope of cross-compilation.