Setup system header wrappers for C++ on VxWorks

Starting from VxWorks 7, the system comes with a Dinkumware
environment which requires the inclusion of "yvals.h" before other
system headers.  We provide wrapped versions of a few headers to
accommodate such constraints.

2019-12-30  Jerome Lambourg  <lambourg@adaccore.com>
            Olivier Hainque  <hainque@adacore.com>

	gcc/
	* config/vxworks/_yvals.h: New file.
	* config/vxworks/_yvals-wrapper.h: New file.
	* gcc/config/vxworks/math.h: Use it to wrap the VxWorks
	math.h header.
	* gcc/config/vxworks/complex.h: Likewise.
	* gcc/config/vxworks/setjmp.h: Likewise.
	* gcc/config/vxworks/inttypes.h: Likewise.
	* config.gcc (*-*-vxworks*): Add system header wrappers
	to extra_headers.
	(powerpc-*-vxworks*): Reuse the common extra_headers.

From-SVN: r279784