Older/legacy Intel GPU OpenCL support

[Mynacol]

Hi, I tried to use OpenCL, but noticed that my Lenovo Thinkpad T480 was seemingly missing the required libraries despite importing its profile. clinfo is returning zero platforms.

Then I realized this affects all "legacy platforms" that try to use the intel-compute-runtime, i.e. Gen 8, 9 and 11 Intel cores. They instead have to use intel-compute-runtime-legacy1. With that I get an expected clinfo output.
I propose a new option in the common intel gpu configuration to switch between those. Installing them side-by-side is not possible due to a file conflict. Specific hardware profiles then should set it correctly to get either of those packages.

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P.S.: The T480 profile should set hardware.intelgpu.vaapiDriver = "intel-media-driver";.

[Mic92]

All you need to do is to include the correct intel generation here:

nixos-hardware/lenovo/thinkpad/t480/default.nix

Line 5 in 34b64e4

../../../common/cpu/intel

We already have abstraction for it:

https://github.com/NixOS/nixos-hardware/blob/master/common/cpu/intel/tiger-lake/default.nix
https://github.com/NixOS/nixos-hardware/blob/master/common/gpu/intel/tiger-lake/default.nix

[Mynacol]

I see no mention of intel-compute-runtime-legacy1 anywhere in this repo. There are abstractions for different Intel CPU/GPU generations, but all import intel-compute-runtime, which doesn't work on older generations. I repeat that my use case is OpenCL, the other stuff like OpenGL, Vulkan, VAAPI works already.
This means the appropriate Intel generations should get this version of intel-compute-runtime, which requires a bit of refactoring I fear. Additionally, the Thinkpad T480 profile should add the correct generation, which is made difficult because it was released with two different Intel generations of CPUs (7th + 8th).