Your project does not have a 'main', which makes sense since you are creating an assembly project.
The issue is that the 'lm4f120h5qr_startup_ccs.c' file and the cmd file that is in your project (generated by CCS) requires the run-time library and the run-time library requires a 'main'.
If you don't need the the generated source file, you can remove it from your project and also the run-time library support from the build options.