radeonsi: add mesa kernel + nir serialised support

This is the basic code to get a opencl kernel using NIR to
run on radeonsi.

Setting AMD_CL_NIR=1 is required to use the NIR path instead
of the current LLVM path.
217 jobs for !7231 with radeonsi-clover-nir in 27 minutes and 28 seconds (queued for 11 seconds)
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