iOS.sh: Update script to include arm64 and current OSX architecture in generated libsodium.a

The current iOS build script targets older iOS devices, this updates the builds script to build a fat lib that includes arm64 support. 

Additionally, the script now produces a build targeting the current system (assuming it's OSX) and merges that with the iOS library so that libsodium can run on both the device and the simulator (for unit testing).