[OpenMP] Use __OPENMP_NVPTX__ instead of _OPENMP in complex wrapper headers.
    
    This is very similar to 7f1e6fcff942, just fixing a left-over.
    With this, it should be possible to use both, -x cuda and -fopenmp in the same invocation,
    enabling to use both OpenMP, targeting CPU, and CUDA, targeting the GPU.
    
    Reviewed By: jdoerfert
    
    Differential Revision: https://reviews.llvm.org/D90415
    
    (cherry picked from commit eaee608448c832e8f806faae30ae4100620c4688)