misc: Add __get_nprocs_sched

This is an internal function meant to return the number of avaliable
processor where the process can scheduled, different than the
__get_nprocs which returns a the system available online CPU.

The Linux implementation currently only calls __get_nprocs(), which
in tuns calls sched_getaffinity.

Reviewed-by: Florian Weimer <fweimer@redhat.com>