BSD compatibility for <machine/endian.h>

Introduce <machine/_endian.h> to let target based customization of
<machine/endian.h> via

 * _LITTLE_ENDIAN,
 * _BIG_ENDIAN,
 * _PDP_ENDIAN, and
 * _BYTE_ORDER.

defines.  Add definitions expected by FreeBSD to
<machine/endian.h> like

 * _QUAD_HIGHWORD,
 * _QUAD_LOWWORD,
 * __bswap16(),
 * __bswap32(),
 * __bswap64(),
 * __htonl(),
 * __htons(),
 * __ntohl(), and
 * __ntohs().

Also, if __BSD_VISIBLE

 * LITTLE_ENDIAN,
 * BIG_ENDIAN,
 * PDP_ENDIAN, and
 * BYTE_ORDER.

Targets that define __machine_host_to_from_network_defined in
<machine/_endian.h> must provide their own implementation of

 * __htonl(),
 * __htons(),
 * __ntohl(), and
 * __ntohs(),

otherwise a default implementation is provided by <machine/endian.h>.
In case of GCC defines to builtins are used.

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

winsup/cygwin/include/endian.h

@@ -12,14 +12,21 @@ details. */
 #define _ENDIAN_H_
 
 #include <sys/config.h>
-#include <sys/cdefs.h>
-#include <bits/endian.h>
+#include <machine/endian.h>
 
 /*#ifdef  __USE_BSD*/
-# define LITTLE_ENDIAN  __LITTLE_ENDIAN
-# define BIG_ENDIAN     __BIG_ENDIAN
-# define PDP_ENDIAN     __PDP_ENDIAN
-# define BYTE_ORDER     __BYTE_ORDER
+# ifndef LITTLE_ENDIAN
+#  define LITTLE_ENDIAN __LITTLE_ENDIAN
+# endif
+# ifndef BIG_ENDIAN
+#  define BIG_ENDIAN    __BIG_ENDIAN
+# endif
+# ifndef PDP_ENDIAN
+#  define PDP_ENDIAN    __PDP_ENDIAN
+# endif
+# ifndef BYTE_ORDER
+#  define BYTE_ORDER    __BYTE_ORDER
+# endif
 /*#endif*/
 
 #if __BYTE_ORDER == __LITTLE_ENDIAN