Add option to disable warnings when ASYNCIFY_REMOVE functions are not found

[curiousdannii]

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.

[kripken]

Sounds reasonable to me.

Looks like there is already an asyncify-verbose option. A "silent" or "no warnings" option could be added similarly.

[sbc100]

All warnings from python code in emscripten should be disable-able via the -Wno-foo syntax. We should probably use the same syntax here and pass that information to binaryen.

Marking as help wanted.