Cygwin: Implement sched_[gs]etaffinity()

This patch set implements the Linux syscalls sched_getaffinity,
sched_setaffinity, pthread_getaffinity_np, and pthread_setaffinity_np.
Linux has a straightforward view of the cpu sets used in affinity masks.
They are simply long (1024-bit) bit masks.  This code emulates that view
while internally dealing with Windows' distribution of available CPUs among
processor groups.

winsup/cygwin/release/3.1.0

@@ -5,6 +5,9 @@ What's new:
   1703 or later.  Add fake 24 bit color support for legacy console,
   which uses the nearest color from 16 system colors.
 
+- New APIs: sched_getaffinity, sched_setaffinity, pthread_getaffinity_np,
+  pthread_setaffinity_np.
+
 
 What changed:
 -------------