C API: Re-expose GC_allow_register_threads

[roberth]

Is your feature request related to a problem? Please describe.

The Rust bindings use GC_allow_register_threads and GC_register_my_thread/GC_unregister_my_thread to make its unit tests work (avoid "collecting from unknown thread").

This limits the usefulness of the bindings in case of future versions or alternate implementations.

Describe the solution you'd like

Bdwgc is an implementation detail, so the C API should provide what are effectively wrappers for those functions, so that we don't restrict Nix when it comes to the choice of GC.

Document the problem solved by these functions.

Apply to the Rust bindings for validation:

-#include <gc/gc.h>

Describe alternatives you've considered

Additional context

Priorities

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