acpi_cpu: Make device unit numbers match OS CPU IDs.

There are already APIC ID, ACPI ID and OS ID for each CPU. In perfect
world all of those may match, but at least for SuperMicro server boards
none of them do. Plus none of them match the CPU devices listing order
by ACPI. Previous code used the ACPI device listing order to number
cpuX devices. It looked nice from NewBus perspective, but introduced
4th different set of IDs. Extremely confusing one, since in some places
the device unit numbers were treated as OS CPU IDs (coretemp), but not
in others (sysctl dev.cpu.X.%location).