[OpenMP] Provide math functions in OpenMP device code via OpenMP variants

For OpenMP target regions to piggy back on the CUDA/AMDGPU/... implementation of math functions,
we include the appropriate definitions inside of an `omp begin/end declare variant match(device={arch(nvptx)})` scope.
This way, the vendor specific math functions will become specialized versions of the system math functions.
When a system math function is called and specialized version is available the selection logic introduced in D75779
instead call the specialized version. In contrast to the code path we used so far, the system header is actually included.
This means functions without specialized versions are available and so are macro definitions.

This should address PR42061, PR42798, and PR42799.

Reviewed By: ye-luo

Differential Revision: https://reviews.llvm.org/D75788