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> Otherwise how does this work at all?

Hopes, prayers, and a single version of a single compiler being involved.

This. The ABI is an aspiration, not an implementation. It has turned out well elsewhere than bitfields.

Most ABIs are complex and only partially specified; you're playing with fire if you use bitfields or other ill-defined features in public APIs.

Never ever use bitfields in structs that may cross library boundaries. There are some corners of C that are not fit for public APIs.