[MacPorts] #56295: Correctly represent 64-bit platform as x86_64 in ${os.arch} and pre-generated PortIndex

[MacPorts]

#56295: Correctly represent 64-bit platform as x86_64 in ${os.arch} and pre-
generated PortIndex
--------------------------+--------------------
  Reporter:  raimue       |      Owner:  (none)
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  ports        |    Version:
Resolution:               |   Keywords:
      Port:               |
--------------------------+--------------------

Comment (by raimue):

 I think this problem currently hits us with librsvg on arm64. The
 PortIndex contains version 2.52.4 for arm64, and is always reported as
 outdated to users that have version 2.40.20 installed. But when the user
 actually tries to upgrade, nothing happens, because parsing the Portfile
 will see version 2.40.20.

 This happens because the Portfile is [conditional
 https://github.com/macports/macports-
 ports/blob/b3d5a287abc43bdd61630c2bef201e2ee3d0a8d4/graphics/librsvg/Portfile#L70]
 on `${build_arch}`, but this option is not spoofed by portindex when
 generating the PortIndex for a non-native macOS version.

-- 
Ticket URL: <https://trac.macports.org/ticket/56295#comment:2>
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