[Bazel] feature flags for the Emscripten toolchain

What is the current state of the feature flags for the Emscripten toolchain? It seems like this is in a bit of a half finished state with a lot of options missing. For example:

1. The `exceptions` feature that adds either `-fno-exceptions` or `-fexceptions`, but no way to set `fwasm-exceptions`.
2. Options like `PROXY_TO_PTHREAD` and `WASM_BIGINT` are completely missing
3. There is a feature called `wasm_warnings_as_errors` which just sets `-Werror` and seems on by default? I mean, this feature has nothing to do WebAssembly or Emscripten and doesn't seem like it should exist in the toolchain at all?

Is there interest in cleaning this up via some PRs?