If a system has the ability to disable some cores, as on
ppc64 with the ppc64_cpu command, then TestSchedSetaffinity will
fail if the CPUset passed to Setaffinity includes one that has
been disabled.

This adds a check to use values from the oldMask, which are the valid
cores returned from Getaffinity, to pass to Setaffinity.

Fixes golang/go#27875

Change-Id: I9656f41867afc18e0eaedc4bdef5f75e137a1fcd
Reviewed-on: https://go-review.googlesource.com/137675Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>