* Add LTO for C++ builds
This didn't have much effect on fastcomp builds, but provides further size savings with new LLVM backend we switched to in #750 (and fixes the MozJPEG size regression from the same PR).
In the future we won't need to pass `--llvm-lto 1` explicitly, but latest Emscripten Docker image doesn't contain the Emscripten version with the necessary fixes for this.
* Delete build.log
Co-authored-by: Jake Archibald <jaffathecake@gmail.com>
Looks like we've been stuck on Emscripten fastcomp backend, misssing out on all new optimisations between LLVM 6 and LLVM 11 and going via slow asm.js pipeline when building.
This changes Docker images for Emscripten projects to point to emscripten-upstream instead and commits the updated artifacts.
