Previously, the bootloader would only provide a single memory range.
At least on VirtualBox, this memory range includes the ACPI tables,
which the kernel then happily overwrites when executing VM. Much of
the infrastructure to use a fullblown memory map is already in place;
this patch adds the last (and strangely missing) bit of generating
the memory map in a multiboot-compatible way.
It should be noted that both the bootloader and the kernel actually
violate the multiboot specification by not packing the structure for
the memory ranges. This is a NetBSD bug, but it is also a nonissue
for (our) practical purposes. It can be fixed without changing the
code added in this patch.