VarargsInvoker should use variadic layouts

VarargsInvoker creates a specialized function descriptor here:

https://github.com/openjdk/jextract/blob/master/src/main/resources/org/openjdk/jextract/impl/resources/RuntimeHelper.java.template#L157

But does mark the function descriptor as a variadic descriptor. As such, the linkage request will misbehave on certain platforms (Windows and MacOS/AArch64).