Move PedArchitecture and ped_set_architecture to private

Move PedArchitecture and ped_set_architecture to private

Code that works with libparted isn't suppose to need to change the
architecture where it's running and then doesn't make sense to export
it.

Two new files has been create (libparted/architecture.[hc]) that has
the PedArchitecture structure and the ped_set_architecture
implementation.  All changes that were need to get it suported on all
currently available architectures has been done too.