[mlir][ROCDL] Set the LLVM data layout when lowering to ROCDL LLVM (#74501)

In order to ensure operations lower correctly (especially
memref.addrspacecast, which relies on the data layout benig set
correctly then dealing with dynamic memrefs) and to prevent compilation
issues later down the line, set the `llvm.data_layout` attribute on GPU
modules when lowering their contents to a ROCDL / AMDGPU target.

If there's a good way to test the embedded string to prevent it from
going out of sync with the LLVM TargetMachine, I'd appreciate hearing
about it. (Or, alternatively, if there's a place I could farctor the
string out to).