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/miscfuncs.h

@@ -120,5 +120,6 @@ extern "C" HANDLE WINAPI CygwinCreateThread (LPTHREAD_START_ROUTINE thread_func,
 void SetThreadName (DWORD dwThreadID, const char* threadName);
 
 WORD __get_cpus_per_group (void);
+WORD __get_group_count (void);
 
 #endif /*_MISCFUNCS_H*/