Proposal: Create `emld` entry point that can be called from clang as a linker (e.g. -fuse-ld=emld)

This would allow clang to drive the emscripten linking process.

Kind of a logical extension of:

https://github.com/emscripten-core/emscripten/issues/9218

The `emld` entry point would work much like `emcc` today except:

1. It would only cover linking
2. It would not accept any compile flags
3. Any unhandled flags would be passed directly through to `wasm-ld`

Point (3) would mean that flags such as `-start-group` and `-end-group` would be accept directly and not via `-Wl,` (as they are in emcc).