Allow setting the OS X SDK to "none" to allow x86_64 to be an iOS build.

This allows the providence of the `x86_64` to be decided by the developer on the command-line. The default behavior remains unchanged, in that `x86_64` is assumed to be a `MaxOSX` platform build. However, setting `--osx-sdk-version none` on the command-line switched `x86_64` to the `iPhoneSimulator` platform, making it suitable for use in a 64-bit simulator.