Use the 'fallthrough' attribute

Use the 'fallthrough' attribute

C23[1] and CPP17[2] introduced the 'fallthrough' attribute to let the
compiler know that we're intentionally falling through a case statement.
This suppress a warning with new compilers when doing an implicit fallthrough.

[1] https://en.cppreference.com/w/c/language/attributes/fallthrough
[2] https://en.cppreference.com/w/cpp/language/attributes/fallthrough