Only export symbols on Linux GNU builds

[Schamper]

This fixes https://github.com/indygreg/PyOxidizer/issues/673.

I also ran into this issue and narrowed it down to this commit: https://github.com/indygreg/PyOxidizer/commit/1ad7fbcf812c230a092389d639dd8ba24d8b7b57

Unfortunately I have no idea why precisely this change breaks musl builds, but reverting these changes somehow makes them work again. Since loading extensions isn't possible on musl builds anyway, I restricted the change in the original commit further to only GNU builds.