Add VxWorks fixincludes hack, open posix API for C++

When system headers expose a strict "open" prototype with
3 args, arrange to expose a C++ overload with only two.

2021-01-10  Olivier Hainque  <hainque@adacore.com>

	* inclhack.def (vxworks_math_h_fp_c99): New hack.
	* tests/base/fcntl.h: Update.
	* fixincl.x: Regenerate.