Support for additional POSIX platforms (FreeBSD, OpenBSD, NetBSD)

[kristjanvalur]

Overview

stackman currently supports Linux, macOS, and Windows. Adding BSD variants would expand platform coverage for POSIX systems.

Platforms to consider

1. FreeBSD - Popular server OS, similar to Linux ABIs
2. OpenBSD - Security-focused, may need special considerations
3. NetBSD - Highly portable, good test of platform abstraction

Technical considerations

• Most use same ABIs as Linux (sysv_amd64, arm64, etc.)
• Main differences: object format details, system headers
• May need adjustments to Makefile and CI workflows
• OpenBSD has W^X restrictions that may affect stack switching

Implementation approach

• Start with FreeBSD (largest user base)
• Likely can reuse existing switch_*_gcc.S files
• Add platform detection in platforms/platform.h
• CI: GitHub Actions has FreeBSD support via vmactions/freebsd-vm

Priority

Medium - These platforms have active user communities but smaller than Linux/macOS/Windows.

Referenced in conversation about future platform additions.