sw:vec_iree: Fix model_input CMake rule

Split the gen_mlmodel_input script into two parts: Download URL and
generate the c-embedded library. In this way, we can add the proper
output file dependency of the CMake rule, so --target clean can clean
the generated files, and any update of the source path can trigger the
rebuild.

Change-Id: I96b89e99f7b722a56be4dea7151c4b8fd495bd7a