Some functionality the different arch backends should support

+ cpu_id() - return the ID of the current process executing the code, with
regular indexing (0 - MAX_CPUS), that is.

+ map_vpage() - map one page according to the specs given.
+ stat_vpage() - get info about mapping at virtual address.
+ mod_vpage() - modify mapping at virtual address.
+ unmap_vpage() - unmap page at virtual address.

+ flush_tlb() - flush TLB cache for running cpu.
+ flush_tlb_all() - flush all TLC caches. (note, flush_tlb is allowed to just
		call flush_tlb_all if the arch doesn't support per-cpu
		flushing.)
+ flush_tlb_region() - ??? maybe?

+ enable_irq() - enable interrupts
+ disable_irq() - disable interrupts

+ probably more stuff, just not entirely sure what parts should go where yet.
+ the backend also has to define a memory layout, of course.