The new renameat2 syscall provides all the functionality of renameat
with an additional flags argument, so make renameat optional so that
future architectures can omit it without getting a warning.

This patch doesn't affect existing architectures.
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: linux-arch@vger.kernel.org