Re-use the emsdk build rules in emscripten-releases?

Currently emscripten-releases essentially duplicates the emsdk build rules for binaryen and llvm (and emscripten itself).

This means that the emsdk build rules can bitrot (at least until @juj comes along and fixes them).

Instead, I propose that we add emsdk to the DEPS (essentially a submodule) of emscirpten-releases, and delete all the duplicated logic from emscripten-releases build.py.

I bet an AI agent could quickly identify any divergences and fix them (any such divergences already represent real issues that are probably worth fixing regardless).