wasm32-unknown-unknown: must not re-enter module after abort

[RalfJung]

@thomcc points out that when we abort on wasm32-unknown-unknown, there's nothing actually stopping whoever invoked that module from calling back into it. That's bad, it leads to unsoundness! We should either find some way to prevent this (I don't know how wasm works, maybe there's a global flag we can set on abort and check on every entry point), or we should at least document this prominently, in some place that people using this target are likely to see.

The wasi and emcscripten targets are not affected, they already "poison" modules on abort so they cannot be re-entered.

[RalfJung]

See here for a good summary of the current situation and some history by @alexcrichton.