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Use `libc::sigaction()` instead of `sys::signal()` to prevent a deadlock Fixes #88585. POSIX [specifies](https://man7.org/linux/man-pages/man3/fork.3p.html) that after forking, > to avoid errors, the child process may only execute async-signal-safe operations until such time as one of the exec functions is called. Rust's standard library does not currently adhere to this, as evidenced by #88585. The child process calls [`sys::signal()`](7bf0736e13/library/std/src/sys/unix/android.rs (L76)), which on Android calls [`libc::dlsym()`](7bf0736e13/library/std/src/sys/unix/weak.rs (L101)), which is [**not**](https://man7.org/linux/man-pages/man7/signal-safety.7.html) async-signal-safe, and in fact causes a deadlock in the example in #88585. I think the easiest solution here would be to just call `libc::sigaction()` instead, which [is](https://man7.org/linux/man-pages/man7/signal-safety.7.html) async-signal-safe, provides the functionality we need, and is apparently available on all Android versions because it is also used e.g. [here](7bf0736e13/library/std/src/sys/unix/stack_overflow.rs (L112-L114)).