startup and genericize it so it can be reused to map other tables as well:
- Add a routine to walk a list of ACPI subtables such as those used in the
APIC and SRAT tables in the MI acpi(4) driver.
- Move the routines for mapping and unmapping an ACPI table as well as
mapping the RSDT or XSDT and searching for a table with a given signature
out into acpica_machdep.c for both amd64 and i386.