Add option to disable warnings when ASYNCIFY_REMOVE functions are not found

My project compiles a number of related apps which make slightly different calls into an ASYNCIFY using library. I've determined there are a number of functions which are safe to include in ASYNCIFY_REMOVE, but not all of the apps call all of the functions. It is a little frustrating to have to manually track for each app which functions are called and which aren't, in order to avoid the warnings Binaryen prints.

It would be nice if there was an option to disable the Binaryen warnings when an ASYNCIFY_REMOVE function is not found.