[Bug target/126782] [Darwin] sys/_types/_rsize_t.h does not define rsize_t with -fmodules

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126782

--- Comment #4 from Drea Pinski <pinskia at gcc dot gnu.org> ---
Maybe it is better just to support __need_rsize_t instead.
It comes from `C11 Annex K` which I am not sure even clang supports fully ...