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
(312dff7b79/include/hip/amd_detail/amd_device_functions.h (L957))
Reviewed by: Matt Arsenault, Artem Belevich, Brian Sumner
Differential Revision: https://reviews.llvm.org/D145343