Auto merge of #1955 - gremlin:update-musl-siginfo, r=JohnTitor
Auto merge of #1955 - gremlin:update-musl-siginfo, r=JohnTitor

Bring the MUSL version of siginfo_t up-to-date.

**Background**

As far as I can tell, the [current MUSL version of `siginfo_t`](https://git.musl-libc.org/cgit/musl/tree/include/signal.h#n97) includes fields like [`si_pid`](https://git.musl-libc.org/cgit/musl/tree/include/signal.h#n108) and [`si_uid`](https://git.musl-libc.org/cgit/musl/tree/include/signal.h#n109).

**Change**

This update unifies `siginfo_t` support between GNU and MUSL.

rust-lang/libc,rust-lang/libc,rust-lang/libc