[libc] Add sigfillset and sigdelset

Summary: Add's `sigfillset` and `sigdelset` which will be used in D76676.

Reviewers: sivachandra, PaulkaToast

Reviewed By: sivachandra

Subscribers: mgorny, MaskRay, tschuett, libc-commits

Differential Revision: https://reviews.llvm.org/D76936