[MacPorts] #54198: upgrade: port:gcc7

[MacPorts]

#54198: upgrade: port:gcc7
---------------------+------------------------
  Reporter:  RJVB    |      Owner:  ryandesign
      Type:  update  |     Status:  accepted
  Priority:  Normal  |  Milestone:
 Component:  ports   |    Version:
Resolution:          |   Keywords:  haspatch
      Port:  gcc7    |
---------------------+------------------------

Comment (by ryandesign):

 The holdup is that updating libgcc to 7 will require the introduction of
 libgcc6 and revbumping all gcc ports to depend on it. Since we have to do
 that, I want to take care of some other issues that also require
 revbumping.

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Ticket URL: <https://trac.macports.org/ticket/54198#comment:13>
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