radeon/llvm: Use LLVM C API for compiling LLVM IR to ISA v2

radeon/llvm: Use LLVM C API for compiling LLVM IR to ISA v2

The LLVM C API is considered stable and should never change, so it
is much more desirable to use than the LLVM C++ API, which is constantly in
flux.

v2:
  - Split target initialization and lookup into separate functions

Reviewed-by: Mathias.Froehlich@web.de