[OpenMP] Use __OPENMP_NVPTX__ instead of _OPENMP in complex wrapper headers.

This is very similar to 7f1e6fcf, 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 eaee6084)