posix: Review ABI of siginfo_t

[avdgrinten]

siginfo_t has lots of additional members on Linux, review whether we need them.

[64]

Not all of the fields in this struct are relevant for every signal, and so it's usually implemented as a union. For ABI purposes what we can do is make a union which includes a padding member that we can expand into if future additions are needed. That's also what musl does, bringing the struct to 128 bytes.