[OpenMP] Add `__CUDA_ARCH__` definition when offloading with OpenMP

Currently we define the `__CUDA_ARCH__` macro only in CUDA mode. This
patch allows us to use this macro in OpenMP-offloading mode when
targeting NVPTX.

Reviewed By: tra, tianshilei1992

Differential Revision: https://reviews.llvm.org/D125256