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.arch_(32|64)_bit

This replaces hardcoding of i386/x86_64 all over the code.
Add Hardware::CPU.universal_archs
Use Hardware::CPU.arch(32|64)_bit in formulae
Use Hardware::CPU.universal_archs in formulae