Add libgc

[TheRealKeto]

This PR adds libgc, The Boehm-Demers-Weiser conservative C/C++ Garbage Collector, per request of @CRKatri.

Checklist

• [x] Have you made sure there aren't any other open Pull Requests for the same update/change?
• [ ] This Pull Request doesn't contain any package additions; it's a small change (e.g README change)
• [x] Have you confirmed this builds & works as intended on an iOS device (if applicable)?
• [x] Have you confirmed this builds & works as intended on a macOS device (if applicable)?