Symbol metadata for dynamic linking

The current (experimental) dynamic linking ABI doesn't support any kind of symbol information other than the imports and exports of a module. It does use a small custom "dylink" section but currently has no per-symbol information.

In order to support dynamic linking along + threading (https://github.com/emscripten-core/emscripten/issues/3494) and in order to support weak symbol references (https://github.com/emscripten-core/emscripten/issues/12826) emscripten needs some way to add flags to symbols. Specifically for dynamic linking we need way to mark a given export is TLS and for weak externals we need to way to mark a given import as WEAK.

I think we have 2 options:

1. Add thing information into the import or export name itself
2. Use a custom section.

(1) sounds unattractive since it involved name mangling. However the number of flags we need to is unlikely to grow much (if at all). We could allow for example the @tls suffix on exports and the @weak suffix on imports.

For (2) we would presumably add to the existing dylink section. However we don't currently part the dylink section of the main executable at runtime so that would be something we would have to start doing. If we go with (2) we would need to invent some kind of dynamic symbol table.

Of course in the long term we want to use module linking, but I don't think there is any support for either TLS or weak symbols in the current or planed module linking proposal.

Start by reading the existing custom "dylink" section conventions and the linked issues on dynamic linking with threading and weak symbol references. The work is done when the project has a decided symbol-metadata design that can mark TLS exports and WEAK imports, with the required runtime handling documented.