POSIX barrier implementation, take 3

The attached patch should address all of the review comments.

Modifed change log:

Newlib:

	* libc/include/sys/features.h (_POSIX_BARRIERS): Define for Cygwin.
	* libc/include/sys/types.h (pthread_barrier_t)
	(pthread_barrierattr_t): Do not define for Cygwin.

Cygwin:

	* common.din (pthread_barrierattr_init)
	(pthread_barrierattr_setpshared, pthread_barrierattr_getpshared)
	(pthread_barrierattr_destroy, pthread_barrier_init)
	(pthread_barrier_destroy, pthread_barrier_wait): Export.
	* include/cygwin/types.h (pthread_barrierattr_t)
	(pthread_barrier_t): Declare.
	* include/pthread.h (PTHREAD_BARRIER_SERIAL_THREAD)
	(pthread_barrierattr_init, pthread_barrierattr_setpshared)
	(pthread_barrierattr_getpshared, pthread_barrierattr_destroy)
	(pthread_barrier_init, pthread_barrier_destroy)
	(pthread_barrier_wait): Declare.
	* thread.h (PTHREAD_BARRIER_MAGIC)
	(PTHREAD_BARRIERATTR_MAGIC): Define.
	(class pthread_barrierattr, class pthread_barrier): Declare.
	* thread.cc (delete_and_clear): New local helper function.
	(class pthread_barrierattr, class pthread_barrier): Implement.
	* miscfuncs.h (likely, unlikely): New macros.

--
VH

winsup/cygwin/miscfuncs.h

@@ -11,6 +11,10 @@ details. */
 
 #ifndef _MISCFUNCS_H
 #define _MISCFUNCS_H
+
+#define likely(X) __builtin_expect (!!(X), 1)
+#define unlikely(X) __builtin_expect (!!(X), 0)
+
 int __reg1 winprio_to_nice (DWORD);
 DWORD __reg1 nice_to_winprio (int &);