CI: Retarget 'upstream' origin, add spirv64 libclc

This MR retargets our 'upstream' origin, so we reuse containers between kusma/mesa as the 'upstream', and its downstream forks, avoiding duplicate builds of - in very particular - LLVM.

This change makes our container images coherent between kusma/mesa and downstream forks, at the cost of potentially being incoherent between kusma/mesa and mesa/mesa: jobs will now no longer pull containers from the real upstream Mesa if required. This is because ci-templates does not have multi-level inheritance following fork networks down. Unless and until this exists, the solution to the problem is to make sure that kusma/mesa has the same set of containers as the real upstream for the pipelines we have not customised. This can be done by doing skopeo copy before a rebase, to pull the containers down from mesa/mesa.

Now that we don't have everyone downstream of us rebuilding LLVM, we bump it again in order to get a spirv64 libclc target, so we can switch to a 64-bit memory model in !83 (merged).