Adjust VxWorks fixincludes hack for mkdir to work for C++

For VxWorks, replace an attempt at providing a posix API for
mkdir via macro by a varargs prototype, which works better for
C++ references like std::mkdir(arg1, arg2).

2021-12-16  Olivier Hainque  <hainque@adacore.com>

fixincludes/
	* inclhack.def (vxworks_posix_mkdir): Refine to expose a
	varargs interface.
	* tests/base/sys/stat.h: Update expected results.
	* fixincl.x: Regenerate.