Begin integration of EXT extensions into the unified specification

[kpet]

• Add a khr+ext spec type to makeSpec and cover in CI
• Document the version and dependencies of all EXT extensions in the XML
• Integrate cl_ext_cxx_for_opencl into the unified specification
• Add placeholder descriptions for all EXT extensions linking to either core/KHR features that supersede old extensions that were never part of the specification or the OpenCL Extensions document.

This enables us to integrate EXT extensions incrementally and integrate future EXT extensions directly into the unified specification.

Change-Id: Ic634ce000ad3ebfb56e56bce91f9c0de3e786383