[MacPorts] #58442: gcc9 (and other gccN) fails to install on Mojave, Xcode-10.2.1

[MacPorts]

#58442: gcc9 (and other gccN) fails to install on Mojave, Xcode-10.2.1
-------------------------+--------------------
  Reporter:  mouse07410  |      Owner:  (none)
      Type:  defect      |     Status:  new
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:
Resolution:              |   Keywords:
      Port:  gcc9        |
-------------------------+--------------------

Comment (by kencu):

 It was changed to +llvm70 again by Jeremy on the last update he did. The
 tools are all the same as xcode 10's; the only real change is which clang
 will get called as the assembler. With the `+xcode` variant, you get the
 native Xcode toolchain. With the `+llvm70` variant, you get our new `as`
 logic.

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Ticket URL: <https://trac.macports.org/ticket/58442#comment:18>
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