vm64: map 2GB of memory space at KERNBASE
Add minimal ACPI support -- locate CPUs, LAPICs, and IOAPICs

This will fail if the BIOS puts the ACPI tables above 2GB physical
because that's all the memory we have mapped at the moment.

This is able to find HT cores, which are not listed in the
BIOS MP tables used by mpinit().
Attempt to use ACPI to configure multiprocessor mode.

Fails back to the BIOS MP table based mpinit() if the ACPI tables
are not found or not usable.