kernel: mmu: support for on-demand mappings

This provides memory mappings with the ability to be initialized in their
paged-out state and be paged in on demand. This is especially nice for
anonymous memory mappings as they no longer have to allocate all memory
at mem_map time. This also allows for file mappings to be implemented by
simply providing backing store location tokens.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>