src: use `visibility("default")` exports on POSIX

Electron uses this because Chromium builds with symbols
hidden by default.

Refs:https://github.com/electron/node/commit/
88b494191c2a5b50b01dab80cd61ba3c0e0fbeb9
Refs: https://github.com/electron/node/commit/
1293d1d7d0c33d3925da11ceccdce4eb2e927a43

PR-URL: https://github.com/nodejs/node/pull/25893


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>