build driver

After codegen to LLVM IR, there's a few other steps that need to be taken to actually create an executable:
1. Run LLVM's codegen backend to convert the IR to target assembly (`llc`)
2. Assemble the target code into an object file (`as` or `gcc -c`)
3. Compile the c driver into an object file (`gcc -c`)
4. Link the target object file and driver into an executable (`ld` or `gcc`)

Steps 3 and 4 can be combined into a single gcc command. Anyway, ideally this would all be done in software, but it's a fair amount of boilerplate code to call LLVM's build pipeline that I'm not focusing on while I'm working on the frontend. For now, it would be best to create a makefile and some documentation that can build any given femto source file, assemble it, and link it with a driver.