The near-mythic dimensions of the Shackleton expedition story have prompted a spate of retellings. But for sheer power to haunt the imagination, to capture the sense of isolation felt by that small band of men when they were trapped on the bottom of the world with no guarantee they would make it out alive, it’s hard to picture anything surpassing Phantom Limb’s “69˚ S. (The Shackleton Project),’’ at ArtsEmerson.