compel --help: don't hardcode arch list

Instead of hardcoding list of architectures to usage(), let's
generate it from the data.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>