[MacPorts] #61435: use_xcode default value is incorrect on macOS 11.0

[MacPorts]

#61435: use_xcode default value is incorrect on macOS 11.0
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  Reporter:  nicst35  |      Owner:  (none)
      Type:  defect   |     Status:  new
  Priority:  Normal   |  Milestone:  MacPorts 2.6.4
 Component:  base     |    Version:  2.6.99
Resolution:           |   Keywords:  bigsur
      Port:           |
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Comment (by ryandesign):

 The dynamic linker cache, like all caches, is a performance enhancement.
 Apple has used it for years on iOS and has decided it's worth bringing to
 the Mac as well. Not keeping duplicate copies of the libraries in the
 filesystem is a disk space savings.

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Ticket URL: <https://trac.macports.org/ticket/61435#comment:9>
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