[Bug c/77650] struct with a nested flexible array followed by another member accepted

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77650

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
So maybe document this as an extension (and specify its semantics -- I'm
curious how the glibc code behaves)