Declare non-standard pthread_yield()

The non-standard pthread_yield() function is available at least on
Cygwin, FreeBSD and glibc.

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>

newlib/libc/include/pthread.h

@@ -278,6 +278,10 @@ int	_EXFUN(pthread_getcpuclockid,
 int	_EXFUN(pthread_setconcurrency, (int new_level));
 int	_EXFUN(pthread_getconcurrency, (void));
 
+#if __BSD_VISIBLE || __GNU_VISIBLE
+void	_EXFUN(pthread_yield, (void));
+#endif
+
 /* Dynamic Package Initialization */
 
 /* This is used to statically initialize a pthread_once_t. Example: