[MacPorts] #58493: cctools: using clang < 5.0 as default assembler generates assembly errors in some cases

[MacPorts]

#58493: cctools: using clang < 5.0 as default assembler generates assembly errors
in some cases
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  Reporter:  kencu    |      Owner:  jeremyhu
      Type:  defect   |     Status:  assigned
  Priority:  Normal   |  Milestone:
 Component:  ports    |    Version:
Resolution:           |   Keywords:
      Port:  cctools  |
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Comment (by cjones051073):

 Currently the logic in the cctools port that picks the clang version to
 use with `as` is tied to the llvm version used to build the port, via the
 variant. This is a reasonable enough way to do it, but technically is not
 required. Instead of hardcoding into the as binary the MP clang version
 matching the llvm variant, we *could* pass in a list of all possible
 reasonable versions, starting with the most recent and working back. i.e.
 the list of all possible llvm variant versions. `as` would then start at
 the beginning of this list and use which either it first finds.... This
 would allow the port to still be built using the llvm 3.x version, but
 still when as is used at runtime pick a more recent clang version.

 Note that this would actually allow the somewhat convoluted logic the gcc
 ports now need to make sure the correct clang version cctools needs is
 installed to be removed, and instead just declare a dependency on a recent
 clang version, say 8.0, safe in the knowledge it will be used as the
 assembler if needed.

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Ticket URL: <https://trac.macports.org/ticket/58493#comment:8>
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