[MacPorts] #65183: rust: Portfile can fail to parse depending on build_arch

[MacPorts]

#65183: rust: Portfile can fail to parse depending on build_arch
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  Reporter:  jmroot  |      Owner:  MarcusCalhoun-Lopez
      Type:  defect  |     Status:  assigned
  Priority:  Normal  |  Milestone:
 Component:  ports   |    Version:
Resolution:          |   Keywords:
      Port:  rust    |
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Comment (by jmroot):

 Yes, `muniversal.architectures` would be empty in any case where
 `build_arch` is empty (normal on platforms other than macOS) or set to an
 arch that is not supported by the current SDK.

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Ticket URL: <https://trac.macports.org/ticket/65183#comment:3>
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