[Bug target/69846] empty struct value fails to pass properly

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69846

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
In C++11 dummy is a POD, and passing non-PODs through varargs is
conditionally-supported anyway (and G++ supports it).