[fuzzing] wasm2c integration (#2772)

This adds support for fuzzing with wabt's wasm2c that @binji wrote.
Basically we compile the wasm to C, then compile the C to a native
executable with a custom main() to wrap around it. The executable
should then print exactly the same as that wasm when run in either
the binaryen interpreter or in a JS VM with our wrapper JS for that
wasm. In other words, compiling the wasm to C is another way to
run that wasm.

The main reasons I want this are to fuzz wasm2c itself, and to
have another option for fuzzing emcc. For the latter, we do fuzz
wasm-opt quite a lot, but that doesn't fuzz the non-wasm-opt
parts of emcc. And using wasm2c for that is nice since the
starting point is always a wasm file, which means we
can use tools like wasm-reduce and so forth, which can be
integrated with this fuzzer.

This also:

Refactors the fuzzer harness a little to make it easier to
add more "VMs" to run wasms in.

Do not autoreduce when re-running a testcase, which I hit
while developing this.

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