Add a function attribute to include in EXCEPTION_CATCHING_ALLOWED list

Managing large codebases including third-party libraries with a single global `-sEXCEPTION_CATCHING_ALLOWED` list is challenging from a build system perspective. It's also not that easy to find the mangled names and with C++ unnamed namespaces I think there might not even be consistent function names between build machines.

Could a function attribute be added so this could be maintained directly in the source? I'm thinking it would be similar to how EMSCRIPTEN_KEEPALIVE can be used alongside `-sEXPORTED_FUNCTIONS` as both are useful in different circumstances.