emscripten: ban incompatible function casts

Emscripten is known to deal badly with function casts to incompatible types ¹. This patch prevents us to accidentally merge harmful UB casts to wasm enabled code. The warning is really strict but is compliant to the standard. Code that wants to use function cast should be disabled from the wasm platform.

1. https://emscripten.org/docs/porting/guidelines/function_pointer_issues.html ↩