java.lang.foreign.Linker documentation mentions about the need for using the promoted types in FunctionDescriptor for a downcall to foreign variadic function. Since the set of promoted types are platform specific, it's impossible to write portable code without an essential API.
Essential API is of the form:
Class/MemoryLayout[] getSupportedVariadicTypes(); (Linker class)

Linker.nativeLinker().getSupportedVariadicTypes() can be used to create an acceptable FunctionDescriptor