[AMDGPU] Emit predefined macro `__AMDGCN_CUMODE__`

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)

Reviewed by: Matt Arsenault, Artem Belevich, Brian Sumner

Differential Revision: https://reviews.llvm.org/D145343