I would not call this a runtime myself. Go's runtime is "stuff Go does while your program is running" which is very different than something like crt0 that does

I would not call this a runtime myself. Go's runtime is "stuff Go does while your program is running" which is very different than something like crt0 that does some initial setup. Either way, this is optional in C. Some setup is done by the OS as well (it's not like the "run time" makes up where the stack is going to be).