Pointer tagging breaks shmem topology

[bgoglin]

shmem topologies use the exact same pointers for topology, objects, everything in multiple processes by mmapping a shared memory segment at the same place in multiple processes. This is likely incompatible with pointer tagging if tags are supposed to be process-specific (or even finer grain). It fails on CHERI as discussed in #617. I think ARM has some pointer tagging features too, no idea if hwloc shmem would fail there too.