Fix bug in wasm2c's tail call optimization codegen

[squk]

The default wasm-rt implementation defines wasm_rt_memcpy as memcpy which expects void*

[sbc100]

Does the old output code even compile?

[keithw]

I'm a little confused -- we pass the tail-call tests, so... this suggests we need some additional testing here. How is this code-path not hit by the current tests?

[keithw]

I guess there are no spec tests for tail-calls that use multi-value? If so that seems like an omission we should fix. @squk , are you up for either:

• adding a test to this repo (of using tail-call with multi-value) that this PR will make green, and/or
• submitting one upstream to the tail-call proposal repo?

If you don't have cycles for this or don't want to, I think we could probably take it from here too, but I'd love to have a test somewhere.