That message suggests you are trying to run 64-bit code on a 32-bit destination. Having reproduced your steps, I see this problem intermittently, but if I clean and re-run it, it works.
Do make sure that you've selected a 64-bit destination ("My Mac 64-bit") and that you're building for 64-bit Intel (and not "Universal" for instance). I do suspect there's an Xcode bug in here since I'm seeing it fail and then succeed if I clean between, but this is not a supported configuration and Apple's not going to help you (i.e. a radar is probably a waste of time).
BTW: I build for 10.5 every day with Xcode 5 without any problems, but I'm explicitly building for 32-bit only and so am probably not bumping into this problem.