[Bazel] Support for feature debug fission in emsdk-bazel-toolchain

This is a feature request to add support for Debug Fission (dwarf-5 in separated `.dwo`/`.dwp` file), in emsdk-bazel-toolchain.

From similar reports I've learned that the emscripten team is not currently maintaining a toolchain for bazel, but maybe someone else has it partly ready?

The motivation for this fr is to improve the developer experience of debugging C++ applications in WebAssembly by reducing linking times and reducing the time needed to parse symbol files by devtools.

References:
- https://developer.chrome.com/blog/faster-wasm-debugging
- https://emscripten.org/docs/porting/Debugging.html#debugging-in-the-browser
- [bazel `--fission` flag](https://bazel.build/docs/user-manual#fission)
- https://gcc.gnu.org/wiki/DebugFission
- [clang `-gsplit-dwarf` flag](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang1-gsplit-dwarf)