mmu: Virtual memory permission flags

In preparation for demand paging, add permission flags to vma. This will
be used by mprotect() and the page fault handler.

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>
x64: nested exceptions

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

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