Missing links to SPIR-V-related KHR extensions

[bashbaug]

A few SPIR-V related KHR extensions are missing from the list of Khronos extension specifications on the registry webpage. We should add links for at least these missing KHR extensions:

• cl_khr_spirv_no_integer_wrap_decoration
• cl_khr_spirv_extended_debug_info
• cl_khr_spirv_linkonce_odr

TBD whether they should link to the placeholder chapter in the extensions specification or to the section of the SPIR-V environment specification. The link to the SPIR-V environment specification might be more useful.

[oddhack]

ISTM these should be integrated into the API spec appendices and link to the SPIR-V extensions underlying them from there. If you like I'll do that PR. Also see related KhronosGroup/OpenCL-Docs#1111