WIP: Use a more type safe alternative for sigaction types

[lewis-belsten-revill]

Description

We cannot safely rely on considering usize and pointers as interchangeable, especially on CHERI where the pointer size is larger than a usize. Using a union is a much safer option and it actually communicates the fact that we interpret these fields differently in different situations.

Sources

E.G: https://git.musl-libc.org/cgit/musl/tree/include/signal.h#n170

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

This would be a disruptive change to crates currently using these types, however it (or something similar) is absolutely necessary in order for us to be compatible on targets where the size of a function pointer is not equal to a size_t. C is obviously less strict about things which appear to not be unions but that are treated as such, however on the C side, at least the type used in these declarations is the maximum of the two types (fn ptr > size_t), whereas we are just currently using size_t. For an example of the changes this implies to other crates, see lewis-belsten-revill/wait-timeout@f5e0175

[rustbot]

⚠️ Warning ⚠️

• Pull requests are usually filed against the main branch for this repo, but this one is against libc-0.2. Please double check that you specified the right target!