Add MacOS.preferred_arch

Replaced the plethora of ternaries we've used all over the place to
determine whether x86_64 or i386 is called for.
Add Hardware::CPU.universal_archs
Use Hardware::CPU.arch(32|64)_bit in formulae
Use Hardware::CPU.universal_archs in formulae
superenv: use Hardware::CPU.universal_archs

This defines the new HOMEBREW_ARCHS environment variable, which is
currently only set during universal builds, so that the tool wrappers
no longer need to hardcode i386/x86_64.