Make sure all file extensions for wasm are correct

[brson]

Per this comment, we only emit .js for bin targets, it's not clear what we emit for dylib, cdylib, and staticlib, but it seems likely they are incorrect.

I think .a is probably right for staticlib that is going to be run through emcc, but maybe dylibs should be .js.

[eholk]

I think the dylib story will depend on how dynamic linking for WebAssembly works. Here is some thinking on the issue, but I don't think it's official or complete yet: https://github.com/WebAssembly/tool-conventions/blob/master/DynamicLinking.md

[steveklabnik]

Triage: i have no idea if this was done or not.

[tlively]

AFAIK, the dynamic linking story for WebAssembly is still a WIP, so I wouldn't expect rustc to be able to produce WebAssembly dynamic libraries. On the other hand, I assume that once these library types are supported, their extensions will all match the standard Linux extensions, so I'm not sure this issue is still important to have around.