[MacPorts] #53854: update gdb port version

[MacPorts]

#53854: update gdb port version
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 Reporter:  mabraham     |      Owner:
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:
 Keywords:               |       Port:  gdb
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 gdb upstream reports that gdb version 7.12.1 is required on OS 10.12
 (sierra) and later

 https://sourceware.org/gdb/wiki/BuildingOnDarwin#Giving_gdb_permission_to_control_other_processes

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Ticket URL: <https://trac.macports.org/ticket/53854>
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