posix: Consolidate Linux fdatasync syscall

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.