Generate OpenCL C feature dictionary

[kpet]

The macros for OpenCL C features are currently defined manually in c/feature-dictionary.asciidoc. Here is an example:

// opencl_c_kernel_clock_scope_sub_group
ifdef::backend-html5[]
:opencl_c_kernel_clock_scope_sub_group: pass:q[`\__opencl_c_<wbr>kernel_<wbr>clock_<wbr>scope_<wbr>sub_<wbr>group`]
endif::[]
ifndef::backend-html5[]
:opencl_c_kernel_clock_scope_sub_group: pass:q[`\__opencl_c_&#8203;kernel_&#8203;clock_&#8203;scope_&#8203;sub_&#8203;group`]
endif::[]

Authoring these manually is tedious and error prone. They should be generated from the XML.

[bashbaug]

One of the scripts in https://github.com/KhronosGroup/OpenCL-Docs/pull/1174 might be useful for generating this in the short term, or for EXT or vendor-specific extensions.