Use Rust union types

[kellda]

For #1020

Replace some types with unions now that rustc supports them.

I also found sigevent.sigev_notify_thread_id, siginfo_t.__data and sigaction.sa_sigaction that I think should be unions, but I wasn't sure.

[rust-highfive]

r? @Amanieu

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[kellda]

Also I had to ignore iffaddrs.ifa_ifu on Linux to pass CI

[Amanieu]

Unfortunately I believe this is a breaking change, which means we can't actually do this until we bump the crate version to 1.0.

cc @JohnTitor

[kellda]

I'm aware that it is a breaking change and that's not a problem for me to wait until a new major version for this to be merged.

[bors]

:umbrella: The latest upstream changes (presumably #1975) made this pull request unmergeable. Please resolve the merge conflicts.

[tgross35]

@kellda main is now intended for libc 1.0. Are you able to rebase this?

@rustbot author

[rustbot]

Some changes occurred in solarish module

cc @jclulow, @pfmooney

[kellda]

I'm unsure how to find out what's wrong with sigval on emscripten...

[kellda]

Btw, I noted (when I originally opened this PR) that the following fields may also be wroth considering turning into unions:

• sigevent.sigev_notify_thread_id
• siginfo_t.{_pad/__data}
• sigaction.sa_sigaction