* return the built kernel/program in llvm_compute::compile_kernel (either ptx code or spir/llvm binary bitcode)
 * opencl_compute: if a platform and its devices can be used w/o any problems, use it (break out of the platform test loop)
 * more progress in llvm_compute::compile_kernel (calls clCreateProgramWithBinary now with the produced spir bitcode from llvm_compute and prints errors)