[OpenMP] Use __OPENMP_NVPTX__ instead of _OPENMP in wrapper headers

Due to recent changes we cannot use OpenMP in CUDA files anymore
(PR45533) as the math handling of CUDA is different when _OPENMP is
defined. We actually want this different behavior only if we are
offloading with OpenMP to NVIDIA, thus generating NVPTX. With this patch
we do not interfere with the CUDA math handling except if we are in
NVPTX offloading mode, as indicated by the presence of __OPENMP_NVPTX__.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D78155