[Darwin, fixincludes] Fix PR83531

[Darwin, fixincludes] Fix PR83531

There is no reasonable chance that the SDKs in question will be re-
issued, so the only viable solution is a fixincludes.

2019-08-18  C.G. Dogan <gcc+cgdogan.00@gmail.com>
	    Iain Sandoe  <iain@sandoe.co.uk>

	PR target/83531
	* inclhack.def (darwin_api_availability): New, strip leading
	underscores from API_XXXX defines.
	* fixincl.x: Regenerate.
	* tests/base/os/availability.h: New file.

Co-Authored-By: Iain Sandoe <iain@sandoe.co.uk>
From-SVN: r274624