API/ABI: reorganize fields in struct hwloc_obj
API/ABI: reorganize fields in struct hwloc_obj

Move symmetric_subtree near children info.
Move userdata to the end.

We broke the ABI in the previous commit already
by removing one field.

** This commit breaks the ABI **

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