Improve Jebel Barkal balancing for smaller maps when there are very few or no elephant stables.

Let every elephant attack a different target, yielding a more uniform attacker distribution.
Spawn elephants at the Wonder too, preventing a deadlock when players have no more units while the buildings are still ignored by soldiers.
Reported by mapkoc at https://wildfiregames.com/forum/index.php?/topic/24153-is-this-fixed/

Increase time until the first attacker wave on nomad slightly.
Make it depend on the difficulty, as suggested by nani.