ac/llvm: set +cumode for radeonsi
radeonsi switched over to CU wavefront execution mode, but didn't tell LLVM. This can lead to shaders requiring too many VGPRs to be executed in CU mode and so cause GPU resets.
Pass along +cumode to LLVM so it properly spills VGPRs.
Fixes: 9d7eab2a ("radeonsi: don't enable WGP_MODE because of high cost of workgroup mem coherency")