x86_64: basic changes for supporting compatibility mode guest (version 2)

This introduces
- a (generic) config setting to turn on compatibility mode support
- support to load a Dom0 32-bit ELF image on 64-bit
- a compatibility guest mode GDT (requires moving the ring 0 selectors to different GDT slots), and the respective selector handling changes
- some initial code path changes to address the needs of a compatibility mode guest