[Doc] Update OpenCL Build Docs for the Khronos OpenCL SDK

[bashbaug]

Summary

The build docs currently say that building the OpenCL backend (with ONEDNN_GPU_RUNTIME=OCL) requires the Intel OpenCL SDK. It would be great to update this documentation to include instructions to build with the Khronos OpenCL SDK. This may simply require documentation updates or it may require minor CMake changes as well.

URLs

https://uxlfoundation.github.io/oneDNN/dev_guide_build_options.html#opencl

OpenCL runtime requires Intel(R) SDK for OpenCL* applications.

Additional details

I suspect OS-provided packages or other vendor SDKs will suffice for building in most cases, so perhaps these should be documented as well. It would still be nice to provide instructions in the build documentation using the Khronos OpenCL SDK, though, when OS packages or vendor SDKs are unavailable or too old.

In case it's helpful, these are the search hints for OpenCL include files (the search hints for libs are similar). Note that ${opencl_root_hints} is populated by the documented OPENCLROOT option:

oneDNN/cmake/FindOpenCL.cmake

Lines 79 to 95 in 0fcfc32

	find_path(OpenCL_INCLUDE_DIR
	NAMES
	CL/cl.h OpenCL/cl.h
	HINTS
	${opencl_root_hints}
	ENV "PROGRAMFILES(X86)"
	ENV AMDAPPSDKROOT
	ENV INTELOCLSDKROOT
	ENV NVSDKCOMPUTE_ROOT
	ENV CUDA_PATH
	ENV ATISTREAMSDKROOT
	ENV OCL_ROOT
	PATH_SUFFIXES
	include
	sycl
	OpenCL/common/inc
	"AMD APP/include")