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

[trzeciak]

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://gcc.gnu.org/wiki/DebugFission
• clang -gsplit-dwarf flag