[Ada] Add system-vxworks7 variants of system.ads files for Vx7

[Ada] Add system-vxworks7 variants of system.ads files for Vx7

Based on the Vx6 versions, using a different link spec to accomodate VxWorks 7
specificities, in particular the ability in some configurations to rely on
.ctor sections to trigger constructors in kernel modules.

2018-05-29  Olivier Hainque  <hainque@adacore.com>

gcc/ada/

	* libgnat/system-vxworks7-ppc-rtp.ads: New file.
	* libgnat/system-vxworks7-ppc-kernel.ads: New file.
	* libgnat/system-vxworks7-e500-rtp.ads: New file.
	* libgnat/system-vxworks7-e500-kernel.ads: New file.
	* libgnat/system-vxworks7-x86-rtp.ads: New file.
	* libgnat/system-vxworks-ppc64-kernel.ads: Rename as ...
	* libgnat/system-vxworks7-ppc64-kernel.ads: and adjust name of
	gnat-crtbe link spec to use the vx7 variant.

From-SVN: r260877