libretro_ctr.a would be the compiled retroarch frontend.
You have to compile it yourself.
You have to use an specific DevkitPro version, retroarch doesn't compile with the latest available. It's available on the libretro git.
You first have to compile the core, then place the output in the retroarch folder then build retroarch itself.
I'm not anywhere near my pc, i can get you some more detailed instructions when i get to, if still needed.