Interpreter: Advertise intended fallthrough cases
GCC >= 7 started being a bit verbose about this. Unfortunately from
inside macros our only option pretty much seems to be
__attribute__((fallthrough));, which we may not want to expose to other
compilers, hence using ADVERTISE_FALLTHROUGH macro.
For the general case, it seems to be the better option to stick with
/* FALLTHRU */ comments which GCC will also accept.