fix the size of `posix_spawn_file_actions_t` and `posix_spawnattr_t` on Linux

[japaric]

fixes #3608

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (or someone else) some time within the next two weeks.

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[pvdrz]

CI is failing due to https://github.com/rust-lang/libc/issues/3613

[JohnTitor]

Could you rebase to pass CI?

[coolbluewater]

@JohnTitor, is this libc release blocked waiting for a response here? How to unblock it?

[japaric]

closing in favor of #3683