rename platform=darwin -> platform={ios,macosx}, fix ios simulator build

As proposed on the mailing list: https://groups.google.com/forum/#!topic/avian/z9gojh3J6fw

Current:
make platform=darwin arch=x86_64 (ios=false)
make platform=darwin arch=i386 (ios=false)
make platform=darwin arch=i386 ios=true # ios simulator
make platform=darwin arch=arm ios=true # real ios device

Proposal (respectively):
make platform=macosx arch=x86_64
make platform=macosx arch=i386
make platform=ios arch=i386 # ios simulator
make platform=ios arch=arm # real ios device