hppa: Fix miscompilation of sched_setaffinity() [BZ #18480]

The attached change fixes the miscompilation of sched_setaffinity() on
hppa.  This is an old problem that was fixed on other architectures using
a similar approach to the attached change.  See:
https://sourceware.org/ml/libc-hacker/2004-04/msg00016.html

Build tested on trunk.  Patch has been applied to debian glibc for some time.

(cherry picked from commit 04ece7d2)