To confirm, you're looking for an interpreter? Because most "C implementations" of sorts would instead compile to a standalone binary. If you are looking for an interpreter, while [0] appears out of date, [1] could easily be consumed statically in a C lib and is basically as portable as a C impl. Even better, compile it at runtime and then run it [2]. There's of course the canonical OCaml one [3]. But it would not be hard to write one, WASM is fairly trivial.

0 - https://github.com/kanaka/wac 1 - https://github.com/paritytech/wasmi 2 - https://github.com/sunfishcode/wasmtime 3 - https://github.com/WebAssembly/spec/tree/master/interpreter