[Ada] Add a new gnat tool vxlink

[Ada] Add a new gnat tool vxlink

VxLink is a helper tool used as a wrapper around g++/gcc to build
VxWorks DKM (Downloadable Kernel Modules).

Such DKM is a partially linked object that includes entry points for
constructors and destructors.

This tool thus uses g++ to generate an intermediate partially linked
object, retrieves the list of constructors and destructors in it and
produces a C file that lists those ctors/dtors in a way that is
understood be VxWorks kernel. It then links this file with the
intermediate object to produce a valid DKM.

2018-08-21  Jerome Lambourg  <lambourg@adacore.com>

gcc/ada/

	* vxlink-bind.adb, vxlink-bind.ads, vxlink-link.adb,
	vxlink-link.ads, vxlink-main.adb, vxlink.adb, vxlink.ads: Add a
	new tool vxlink to handle VxWorks constructors in DKMs.
	* gcc-interface/Makefile.in: add rules to build vxlink

From-SVN: r263736