move make_kernel to backend

> We are ok with this PR. Would also like to add that the make_kernel path is being used for OpenCL interop. In general, the make_ functions are extremely important for us and should remain in the core spec.

Would you be opposed to moving them into a dedicated OpenCL namespace? The deprecation path I see is something like:

- No change to SYCL 2020.
- In SYCL-Next, `sycl::make_kernel` is deprecated in favor of something like `sycl::opencl::make_kernel`.
- In SYCL-Next-Next, `sycl::make_kernel` is removed.

So, it wouldn't be removed any time soon, there'd be a simple migration path (just a new namespace) and we'd be free to let each backend define its own interoperability interfaces.

_Originally posted by @Pennycook in https://github.com/KhronosGroup/SYCL-Docs/issues/754#issuecomment-2755038927_

The issue names no file, test, or entry point. Begin by reviewing the specification's treatment of sycl::make_kernel and OpenCL interoperability, then clarify the proposed namespace and deprecation plan across SYCL-Next versions. Done requires an agreed specification change and migration path.

Written by the indexing model from the issue text.