Isn't libc just a set of libraries and therefore not considered a runtime environment? C does have a small runtime environment, but I'd say it's the existence of the function call stack, as well as the on_exit system that's built into the core language.
As well as things like floating point and large integer emulation, startup code, etc. __divdi3, for example, divides 64 bit integers on i386.
See /usr/lib/x86_64-linux-gnu/{crt1.o,crti.o,crtn.o,gcrt1.o,Mcrt1.o,Srct1.o}.
I think your parent comment means runtimes in the sense of a garbage collector or other services that run concurrently with user code or are scheduled periodically, which will put you in a world of hurt if you touch one of their objects without asking its permissions and/or playing by its rules.
Well, it's so small compared to "real" runtimes, it might as well not exist.