Summit: Work-Around IBM MPI Collectives (#2283)

Fix crashes on Summit at scale (>~224 nodes) due to failing Barriers
in IBM's MPI stack.

Seen mostly with I/O routines and only since the RHEL8 upgrade.
We see no signfiicant performance impact from this work-around until
OLCF & IBM fix the problem (OLCFHELP-3545).

An alternative work-around via
```
export OMPI_MCA_coll_ibm_collselect_mode_barrier=failsafe
```
was tested and is a tiny bit slower than just falling back to HCOLL
or OMPI's barrier implementations via
```
export OMPI_MCA_coll_ibm_skip_barrier=true
```

Thanks to Brian Smith at OLCF for the support!

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