Reduce modifications done in wasm-emscripten-finalize #3043
Description
Currently wasm-emscripten-finalize both writes out some metadata and modifies the wasm. If we can reduce the amount of modifications, we can eventually get to a place where there are no modifications, at least in some build types, which would be simpler and faster.
The larger goal here is to let emscripten do nothing after wasm-ld links the wasm, that is, not modify the wasm at all. Optimized builds will still want wasm-opt, of course, but it would be nice if unoptimized ones didn't need to.