Adding Stockfish 5 recipe

Stockfish 5 is the strongest chess engine in the world. The latest release just won the TCEC tournament.
`MacOS.prefer_64_bit?` rather than `Hardware.is_32_bit?`

(There are a couple of older platforms that are still supported where building 64-bit is iffy, and is either positional or disabled outright even on 64-bit capable hardware.)