arch -> uname -m in Makefile

[ashiklom]

Summary

Minor thing, but arch is less likely to be present on a system than uname -m (case in point: It was missing in my WSL2 Arch Linux install). I think they return basically the same information.

How to test

Steps to reproduce and verify the change locally:

make sipnet

Related issues

None

Checklist

• Tests added for new features
• Documentation updated (if applicable)
• docs/CHANGELOG.md updated with noteworthy changes
• Code formatted with clang-format (run git clang-format if needed)
• Requested review from at least one CODEOWNER

For model structure changes:

• Removed \fraktur font formatting from docs/model-structure.md for implemented features

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Note: See CONTRIBUTING.md for additional guidance. This repository uses automated formatting checks; if the pre-commit hook blocks your commit, run git clang-format to format staged changes.