mmu: support MAP_UNINITIALIZED flag

When seeing this flag, pages fault in should not be filled with zeroes or any
other patterns, and should rather be just left alone in whatever state we find
them at.

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