Cannot serialize type kind Opaque with valists and wrap-static-fns

[NickeZ]

Test fails for aarch64-unknown-linux-gnu.

How to reproduce:

$ cargo run -- --wrap-static-fns bindgen-tests/tests/headers/wrap-static-fns.h -- -target aarch64-unknown-linux-gnu
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.08s
     Running `target/debug/bindgen --wrap-static-fns bindgen-tests/tests/headers/wrap-static-fns.h -- -target aarch64-unknown-linux-gnu`
Unable to generate bindings: codegen error: serialization error at bindgen-tests/tests/headers/wrap-static-fns.h:63:56: Cannot serialize type kind Opaque

Was found when trying to enable that target in CI: #3217

There are some more hints in the discussions in that PR.