The Garden of Eden isn’t big enough for three people, it seems. Chiwetel Ejiofor, Margot Robbie and Chris Pine’s characters find that out the hard way in “Z for Zachariah,” an adaptation of a posthumously-published novel by Robert C. O’Brien (“Mrs. Frisby and the Rats of Nimh”). It’s set in the aftermath of unspecified decline-and-fall that definitely included nuclear weapons; pockets of radiation are everywhere, even in remote rural areas, and the film includes sequences where radiation-proof suits are used and people talk about underground bases and protocols. It’s hard enough just to survive out here, or so we’re told; forget about rebuilding civilization.