linux: only use the device-tree on Power machines
linux: only use the device-tree on Power machines

It's available on ARM but the assumption tat cpus' "reg" start at 0
is invalid.
We could make that work but the device-tree doesn't currently
bring anything better than sysfs on ARM, so don't bother for now.

Refs #127

shekkbuilder/hwloc,shekkbuilder/hwloc,shekkbuilder/hwloc,shekkbuilder/hwloc