SchedGetaffinity and SchedSetaffinity are used to get and set a thread's
CPU affinity mask.

See http://man7.org/linux/man-pages/man2/sched_setaffinity.2.html for
details.

Also add a manual implementation of CpuSet_t and the corresponding
accessor functions (mimicking the CPU_* macros from sched.h).

Fixes golang/go#11243

Change-Id: Ia5abc0053cd06810b3b09ab65c27434f5323c1ad
Reviewed-on: https://go-review.googlesource.com/85915
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>