[MacPorts] #54289: libcxx{,-abi} : upgrade + "runtime" variant

[MacPorts]

#54289: libcxx{,-abi} : upgrade + "runtime" variant
-------------------------------+----------------------
  Reporter:  RJVB              |      Owner:  jeremyhu
      Type:  update            |     Status:  assigned
  Priority:  Normal            |  Milestone:
 Component:  ports             |    Version:
Resolution:                    |   Keywords:  haspatch
      Port:  libcxxabi libcxx  |
-------------------------------+----------------------

Comment (by RJVB):

 The principle of a runtime variant is still as relevant as it ever was, of
 course.

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Ticket URL: <https://trac.macports.org/ticket/54289#comment:16>
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