Predefine __AMDGCN_CUMODE__ as 1 or 0 when compilation assumes CU or WGP modes.
If WGP mode is not supported, ignore -mno-cumode and emit a warning.
This is needed for implementing device functions like __smid (https://github.com/ROCm-Developer-Tools/hipamd/blob/312dff7b794337aa040be0691acc78e9f968a8d2/include/hip/amd_detail/amd_device_functions.h#L957)
I'm surprised there already isn't a warning for this?