"Native" emscripten build environment

Emscripten should figure out a way to provide an execution environment with the following features:

- posix_spawn/system, using filesystem emulation and unsafe-eval if needed (to load wasm modules from the emulated filesystem).

This would enable building and running thngs like make and uname *in* emscripten, which in turn plays nicer with build systems, which in turn makes it possible to build more stuff. And because it's not seen as cross-compilation, the build system can avoid making assumptions such as whether fork works, and it can also generate artifacts as needed (e.g. libgcrypt/libgpg-error need to generate some stuff on the host machine, which isn't really possible with cross-compilation).