OCL: added runtime flags for debugging

Added environment parameters to control execution of OpenCL code:
- `OPENCV_OPENCL_RAISE_ERROR` - call `CV_Assert` to validate success of OpenCL runtime calls (raise on error);
- `OPENCV_OPENCL_PERF_CHECK_BYPASS` - to bypass performance checks and call OpenCL optimization in the most cases (usage example in arithm.cpp & stat.cpp files);
ocl.hpp is sort of public OpenCV header. It should probably be CV_OCL_PERFORMANCE_CHECK()