Structural improvements (LLVM kernel languages)

[cadivus]

This PR fixes installing LLVM and using the installed version.
The relative were broken if using installed LLVM, resulting in errors like this:

In file included from main.cu:4:
/tmp/installed_software/llvm/include/offload/cuda/cuda_runtime.h:14:10: fatal error: '../../kernel/include/DefineLanguageNames.inc' file not found
   14 | #include "../../kernel/include/DefineLanguageNames.inc"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated when compiling for gfx90a.

Now, this works. If you don't want to do an extra -I on compiling, you can now include

#include <offload/cuda/cuda_runtime.h>

instead of

#include <cuda_runtime.h>

The structure of installed files is:

$ tree include/offload/
include/offload/
├── cuda
│   └── cuda_runtime.h
├── hip
│   └── hip_runtime.h
├── kernel
│   ├── DefineLanguageNames.inc
│   ├── LanguageRuntime.h
│   └── UndefineLanguageNames.inc
├── OffloadAPI.h
└── OffloadPrint.hpp

4 directories, 7 files

[kevinsala]

LGTM