lib/posix-process: Fix integer overflow in sigset operations

lib/posix-process: Fix integer overflow in sigset operations

Fix an integer overflow caused by missing `UL` suffix on
shifted values that would result into incorrect evaluation
when signal numbers >= SIGRTMIN are passed to paremeters.

Signed-off-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Ioan-Teodor Teugea <ioan_teodor.teugea@stud.acs.upb.ro>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
Approved-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1248