adjust d_size_t for OSX 64 bit

[WalterBright]

Because of:

Undefined symbols for architecture x86_64:
  "Identifier::idPool(char const*, unsigned long)", referenced from:
      test_visitors() in cxxfrontend.o
      test_semantic() in cxxfrontend.o

https://auto-tester.puremagic.com/show-run.ghtml?projectid=1&runid=3165089&isPull=true

[dlang-bot]

Thanks for your pull request, @WalterBright!

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Testing this PR locally

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dub fetch digger
dub run digger -- build "master + dmd#8248"

[WalterBright]

Abandoning this. Fixing size_t on OSX is whack-a-mole absurdity. Replacing with #8249