Removing bitcode linking from emscripten

[Sam Clegg]

;tldr; We are hoping to deprecate and remove the bitcode linking (via llvm-link) feature from emscripten.  If you don't use this feature you can stop reading now.

This feature uses `llvm-link` under the hood to combine bitcode/LTO object files into larger bitcode/LTO object files before they are finally linked into an executable.   These days we prefer the more standard methods of using `emar` to combine objects into archives, or using `emcc -r` to combine native (non-LTO) objects into larger ones.

The only way to access this feature today is to pass `-r` *and* `-flto` flags tp `emcc` at the same time.  If you are using this feature and wish to continue to use it please join the conversation on https://github.com/emscripten-core/emscripten/issues/13492.

cheers,

sam