Move arc4random Cygwin only code to Cygwin

Keep the Newlib arc4random.c identical to the OpenBSD upstream version.

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

winsup/cygwin/libc/arc4random_stir.c

@@ -0,0 +1,24 @@
+/* libc/arc4random_stir.c
+
+   Copyright 2016 Red Hat, Inc.
+
+This file is part of Cygwin.
+
+This software is a copyrighted work licensed under the terms of the
+Cygwin license.  Please consult the file "CYGWIN_LICENSE" for
+details. */
+
+#include <sys/types.h>
+
+/* Exported functions removed from OpenBSD in the meantime.  Keep them,
+   but make them non-functional.  They don't return a value anyway. */
+
+void
+arc4random_stir(void)
+{
+}
+
+void
+arc4random_addrandom(u_char *dat, int datlen)
+{
+}