CUDA-Kernels instead of OpenACC

@jahns Currently packing/unpacking kernels for the GPU support are generated using OpenACC. Example for packing 8-Byte-data:

static void xt_ddt_pack_8(
  size_t count, ssize_t *restrict displs, void const *restrict src,
  void *restrict dst, enum xt_memtype memtype) {
XtPragmaACC(
  parallel loop independent deviceptr(src, dst, displs)
  if (memtype != XT_MEMTYPE_HOST))
  for (size_t i = 0; i < count; ++i)
    ((int8_t*)dst)[i] = *(int8_t*)((unsigned char *)src + displs[i]);
}

Alternatively we could write the kernels in CUDA-code and compile them at runtime using NVRTC. This approach is a little bit more complex for us, but the advantages would be:

• no dependencies on OpenACC
• we could compile at runtime for the architecture that is actually being used
• the configure would not have to determine any compiling/linking flags for the CUDA support (or provided by the user); the CUDA-root directory would be sufficient

assigned to @k202077

changed the description

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