sys/cdefs.h: Define __hidden as empty on Cygwin

Non-default visibility attributes are unsupported on PE/COFF, so don't
use in __hidden definition for Cygwin.  Add comment.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

newlib/libc/include/sys/cdefs.h

@@ -466,8 +466,13 @@
 #if __GNUC_PREREQ__(4, 0)
 #define	__sentinel	__attribute__((__sentinel__))
 #define	__exported	__attribute__((__visibility__("default")))
+/* Only default visibility is supported on PE/COFF targets. */
+#ifndef __CYGWIN__
 #define	__hidden	__attribute__((__visibility__("hidden")))
 #else
+#define	__hidden
+#endif
+#else
 #define	__sentinel
 #define	__exported
 #define	__hidden