[libclc] Base the build around `add_sources` instead of source list (#197034)

Summary:
The current build uses a curated + deduplicated source list. This PR
seeks to simplify this a little bit and canonicalize the behavior.

Now we create the target up-front, `clc` and `opencl`. We add the
directories which add sources to this target. We normalize the
architecture to the variants. We always add target specific versions
first. When we add sources we check if the file already exists and defer
to the architecture specific one.

This normalized the behavior, the directories are now laid out like this
`clc/<arch>/<os>`. We normalize these to `amdgpu`, `nvptx`, and `spirv`
respectively. We use the OS for the newly created vulkan target. We now
control variants via checking if the directory for that exists, so it's
nested more naturally.

Hopefully this makes more sense, the goal is to exercise the fact that
we have individual builds now. Previously this did not work because you
could not add_subdirectory more than once.