- fix GLOBAL_SUM_SINGLECPU when using Exch2; re-use same buffers and same   gather/scatter S/R as with SingleCpuIO (=> 1 less 2D global RL array).
- fix GLOBAL_SUM_SINGLECPU when using Exch2; re-use same buffers and same
  gather/scatter S/R as with SingleCpuIO (=> 1 less 2D global RL array).

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