posix: implement `sigpending()`

[ycsin]

There are currently 20 missing POSIX functions with trivial implementations, sigpending() is one of them.

sigpending - examine pending signals

The sigpending() function shall store, in the location referenced by the set argument, the set of signals that are blocked from delivery to the calling thread and that are pending on the process or the calling thread.

See https://pubs.opengroup.org/onlinepubs/9699919799/functions/sigpending.html

See RFC #51211 for more info.