[issue23968] rename the platform directory from plat-$(MACHDEP) to plat-$(PLATFORM_TRIPLET)

[Ned Deily]

Changes by Ned Deily <nad at python.org>:


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priority: normal -> critical
stage:  -> needs patch
versions: +Python 3.6 -Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue23968>
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