unix: add SchedGetaffinity and SchedSetaffinity on Linux

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>

unix/linux/types.go

@@ -24,6 +24,7 @@ package unix
 #include <netinet/tcp.h>
 #include <netpacket/packet.h>
 #include <poll.h>
+#include <sched.h>
 #include <signal.h>
 #include <stdio.h>
 #include <sys/epoll.h>
@@ -594,3 +595,12 @@ const (
 	CTRL_ATTR_MCAST_GRP_NAME   = C.CTRL_ATTR_MCAST_GRP_NAME
 	CTRL_ATTR_MCAST_GRP_ID     = C.CTRL_ATTR_MCAST_GRP_ID
 )
+
+// CPU affinity
+
+type cpuMask C.__cpu_mask
+
+const (
+	_CPU_SETSIZE = C.__CPU_SETSIZE
+	_NCPUBITS    = C.__NCPUBITS
+)