[MacPorts] #62807: openmpi/mpich: prune target compiler list; refactor duplicated logic, and simplify ports, via new portgroup

[MacPorts]

#62807: openmpi/mpich: prune target compiler list; refactor duplicated logic, and
simplify ports, via new portgroup
----------------------------+----------------------
  Reporter:  mascguy        |      Owner:  mascguy
      Type:  enhancement    |     Status:  assigned
  Priority:  Normal         |  Milestone:
 Component:  ports          |    Version:
Resolution:                 |   Keywords:
      Port:  openmpi mpich  |
----------------------------+----------------------

Comment (by mascguy):

 Of note, we also need to add subports for GCC 11 to both `openmpi` and
 `mpich`. While not a top priority, we'll try to do that as part of this
 effort, if possible.

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Ticket URL: <https://trac.macports.org/ticket/62807#comment:16>
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