What does the OS being written in C matter? libc has to make a syscall to access OS functions just like anything else does. There's no obligation on a language implementation to make calls through C. It's a thin enough layer that if your language runtime is otherwise written in C you may as well, but that's a design decision.
It would be perfectly reasonable to write a non-C language runtime targeting Linux against syscall instructions or against Windows' documented system DLL interfaces.
Even if it didn't there's no reason to ever add a C dependency to your system if one of the languages you're already using has sufficient "systems" versatility. Go is clearly intended to fill this role.
tl;dr: you can get past C just fine even on an OS written in C and the C dependency isn't free.
Well, when you make a syscall, there's C on the other side. You can make a syscall without C, but it's not portable to different architectures. I think that is essentially the reason that every language runtime I know of uses C -- it's the only choice to portably access operating system services. If there is a counterexample that would be interesting to know, but I don't think it exists.
Probably Forth could be thought of an alternative non-C stack, and of course it's used on machines without an OS. But that's how far afield you have to go to get away from C.
My point is that when you're using Go your stack is still Go and C, to an extent. Usually you won't need to peek under the hood, but at some point when developing nontrivial services, you always need to. Basically my philosophy is that you should always understand at least 1 abstraction level below what you program in. With either Go or Python, that's C.
It would be perfectly reasonable to write a non-C language runtime targeting Linux against syscall instructions or against Windows' documented system DLL interfaces.
Even if it didn't there's no reason to ever add a C dependency to your system if one of the languages you're already using has sufficient "systems" versatility. Go is clearly intended to fill this role.
tl;dr: you can get past C just fine even on an OS written in C and the C dependency isn't free.