Backend is the correct term: C is by no mean an IR which means "intermediate representation", from which you can more or less easily understand the thing behind the code (eg: types, control flow, variable bindings). An IR is a data-structure that obey some rules, a C file is flat text that needs parsing. Not the same.

If C is the output of the compiler, then we speak of a C Backend, which means the part that transform the final IR in the other language we want to translate to (eg: x86, arm6, sparc, ... C :)

As a final note, it's likely that platforms so conservatives that they only provide a C ansi compiler, will not be aroused by Rust.