Add a script to cross compile cargo

[japaric]

As with #11, this should allow specifying the ABI via a target flag. Again, I can only test the gnueabihf ABI, I need testers for the other ABIs.

The biggest obstacle would be that cargo dynamically links to OpenSSL, so the ARM version of that library will be needed on the host machine. Therefore, a debian unstable chroot should be used, since debian supports multiarch configurations.

[japaric]

Cross compiling cargo is a headache because of the native (C) libraries! The last ARM cargo binary I built from scratch was done by manually running the cargo build --verbose output (commands) in my ODROID XU. Which needless to say is laborious and not really automatable.

I don't think I'll provide a script to cross compile cargo any time soon, so I'm tagging this wontfix.