We had a test claiming that this empty struct type consumes a register
slot when passing it to a function with GCC, but that does not appear to
be the case, at least with GCC versions going back to 4.8.
This also caused a miscompilation when passing one of these structs to a
variadic function, but it turned out that our implementation of `va_arg`
matches GCC's ABI, so the one change fixes both bugs.
e9c2e0acd7