net/openmpi3: Use the embedded hwloc library to workaround a conflict between hwloc and hwloc2 due sysutils/slurm-wlm.

OpenMPI3 is not compatible with hwloc2. Also, OpenMPI3 was deprecated by upstream and will be removed from the ports tree as soon as all the ports which depend on it are moved to net/openmpi.

PR: 252496