mmu: Separate page table population and allocation

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
mmu: Add permission tracking to vma

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
x64: Page fault error code

Add page fault error code constants and pass the error code to
page_fault() function from entry.S.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
mmu: Use on-demand paging for anonymous virtual memory

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>