add openat() to syscall list

We'll need this for use in the restorer blob for restoring LSMs. It looks like
arm already has openat, so I think it's just x86 and ppc that need it. In any
case, please double check this, as I've only tested it on x86.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>