posix: Consolidate Linux fdatasync syscall

This patch consolidates the fdatasync Linux syscall generation on
sysdeps/unix/sysv/linux/fdatasync.c.  It basically removes it from
architectures auto-generation list.

Checked on i686-linux-gnu, x86_64-linux-gnu, x86_64-linux-gnux32,
arch64-linux-gnu, arm-linux-gnueabihf, powerpc64le-linux-gnu,
sparc64-linux-gnu, and sparcv9-linux-gnu.

	* misc/makefile (CFLAGS-datasync.c): New flag.
	* nptl/makefile (CFLAGS-datasync.c): Likewise.
	* sysdeps/unix/sysv/linux/syscalls.list (fdatasync): Remove from
	auto-generation syscall list.
	* sysdeps/unix/sysv/linux/fdatasync.c: New file.

sysdeps/unix/sysv/linux/fdatasync.c

@@ -0,0 +1,29 @@
+/* Synchronize a file's in-core state with storage device Linux
+   implementation.
+   Copyright (C) 2017 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/>.  */
+
+#include <unistd.h>
+#include <sysdep-cancel.h>
+
+/* Synchronize at least the data part of a file with the underlying
+   media.  */
+int
+fdatasync (int fd)
+{
+  return SYSCALL_CANCEL (fdatasync, fd);
+}