tst-getcpu: decode errno when sched_getcpu fails

The tst-getcpu test is failing on ia64 platforms due to sched_getcpu
returning an error.  But the current test only considers -1 when it's
also ENOSYS.  Tweak the logic a bit to make the output clearer.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>