XPUBackend refactoring to facilitate arch-specific implementations

 - Allow metaclasses injection: use XPUBackendMeta from the .meta submodule as the metaclass, if it exists.
 - Try to find an arch-specific implementation in the .arch.<name> submodule.
 - Create the list of passes in separate methods to allow subclasses to modify it.

Author: AndreyPavlenko