Cabal shouldn't try to build executables for iOS

Hi all,
I'd like to disable building of executables when compiling for iOS, since ghc-ios isn't set up to compile binaries (which wouldn't be runnable anywhere anyway... Crypto and pretty-show are the two I'm having trouble with).

A --disable-executables flag would be perhaps a more general approach that I could add to the cabal-ios scripts.

I'm happy to put together the patch — it would be super helpful if someone could give me some pointers on where to look though.