Load full DTB address in ARM64 loader instead of just offset

This alignes DTB loading with other architectures as well as
simplifies DTB address processing in machdep.c
Allow for using static (embedded) DTB in ARM64 kernel

When there is no DTB passed by the loader, try to use
the built-in one that might be present when FDT_DTB_STATIC
option is used.
Fix ARM64 pmap.c compilation when INVARIANTS are enabled

Some minor improvements to make KASSERTs work and more
specific about errors.
Add missing pmap_page_is_mapped() used in KASSERTs in
swap_pager.c, vm_object.c, vm_page.c, vm_pageout.c and vnode_pager.c
Fix typo in address space length