Investigate removing EMULATE_FUNCTION_POINTER_CASTS

[samwillis]

Emscripten option EMULATE_FUNCTION_POINTER_CASTS is currently required to compile as Postgres makes usage of function pointer casts. However this is a potential performance penalty. We should investigate a route to removing the need for this option.

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

Use EMULATE_FUNCTION_POINTER_CASTS. When you build with -sEMULATE_FUNCTION_POINTER_CASTS, Emscripten emits code to emulate function pointer casts at runtime, adding extra arguments/dropping them/changing their type/adding or dropping a return type/etc. This can add significant runtime overhead, so it is not recommended, but is be worth trying.