WebAssembly build of libffi (NOT FINISHED)

SOURCES: The two files in libffi-emscripten are modified versions of files I found at https://github.com/hoodmane/libffi-emscripten

WARNING: This to actually not done yet!

There are several functions in src/libffi-emscripten/src/wasm32/ffi.c that need to be re-implemented to work in the context of CoWasm. Right now they are stubs that print that they are stubs.