Extending NaplesPU for OpenCL support

This section describes how the OpenCL C kernel compilation is supported in the nu+ toolchain. Since Clang natively supports the IR generation of OpenCL C kernels, our work focused on the adaption of the generated code on the nu+ core. As the nu+ start routine requires to jump to the main label after having initialized all necessary resources, it is necessary to build a custom optimization pass, to auto-generate the main-function and to define the calling frame to the kernel function. Also OpenCL requires the implementation of custom libraries, extending the libc set.