extras/tools/libtool: Add sideloading info for emscripten-host

Currently libtool.m4 does not have information about wasm32-unknown-emscripten in host_os. Which causes failure in enabling shared libraries.

This patch will create a new host_os case named emscripten in libtool.m4 to allow us to enable shared library compilation and add emscripten specific shared library building flags. This patch will also add some shared library configuration in libtool for emscripten host.