scripts/checksyscalls.sh: Make renameat optional

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