Revert #63649 - "Upgrade Emscripten targets to use upstream LLVM backend"
This change caused the runtime of the linux-asmjs builder to nearly double from 2+ hours to about 4 hours, which happens to be the bors timeout. (It made it in barely under 4 hours when it was merged.) This is causing timeouts on all new changes.
This reverts commit 7870050796e5904a0fc85ecbe6fa6dde1cfe0c91, reversing
changes made to 2e7244807a7878f6eca3eb7d97ae9b413aa49014.