Upgrade emsdk to 3.1.8

[abhi-agg]

Fixes https://github.com/browsermt/bergamot-translator/issues/404

Changes:

• Upgraded emsdk to 3.1.8
• Followed https://github.com/emscripten-core/emscripten/blob/main/src/settings.js and https://emscripten.org/docs/tools_reference/emcc.html# for compile and link flags
• Updated marian-dev submodule

Inspired by @jelmervdl's https://github.com/browsermt/bergamot-translator/pull/399

[andrenatal]

@abhi-agg is on PTO and requested this to be merged after approved