linux: Use rseq area unconditionally in sched_getcpu (bug 31479)

Originally, nptl/descr.h included <sys/rseq.h>, but we removed that
in commit 2c6b4b27 ("nptl:
Unconditionally use a 32-byte rseq area").  After that, it was
not ensured that the RSEQ_SIG macro was defined during sched_getcpu.c
compilation that provided a definition.  This commit always checks
the rseq area for CPU number information before using the other
approaches.

This adds an unnecessary (but well-predictable) branch on
architectures which do not define RSEQ_SIG, but its cost is small
compared to the system call.  Most architectures that have vDSO
acceleration for getcpu also have rseq support.

Fixes: 2c6b4b27
Fixes: 1d350aa0
Reviewed-by: Arjun Shankar <arjun@redhat.com>
(cherry picked from commit 7a76f218)
Fixes: 9da81743