Add VxWworks fixincludes hack, don't expect yvals.h from gcc

yvals.h on VxWorks expects the toolchain to provide its own
version of the header, which we don't do. Arrange to fallback
on the common system definitions instead.

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

fixincludes/
	* inclhack.def (vxworks_next_yvals): New hack.
	* tests/base/yvals.h: New expected test result.
	* fixincl.x: Regenerate.