<sys/stat.h>: Use Linux UAPI header for statx if available and useful

This will automatically import new STATX_* constants.  It also avoids
a conflict between <sys/stat.h> and <linux/stat.h>.

sysdeps/unix/sysv/linux/bits/statx.h

@@ -0,0 +1,34 @@
+/* statx-related definitions and declarations.  Linux version.
+   Copyright (C) 2018-2019 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+/* This interface is based on <linux/stat.h> in Linux.  */
+
+#ifndef _SYS_STAT_H
+# error Never include <bits/statx.h> directly, include <sys/stat.h> instead.
+#endif
+
+/* Use the Linux kernel header if available.  */
+#if __glibc_has_include (<linux/stat.h>)
+# include <linux/stat.h>
+# ifdef STATX_TYPE
+#  define __statx_timestamp_defined 1
+#  define __statx_defined 1
+# endif
+#endif
+
+#include <bits/statx-generic.h>