radeonsi: Set datalayout on the llvm module

radeonsi: Set datalayout on the llvm module

This prevents LLVM from using sext instructions for local memory offsets
and allows the backend to fold immediate offsets into the instruction.

This also prevents some incorrect code generation for ptrtoint and
inttoptr instructions.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>