syntax for handling variadic ccalls correctly for complex cases

[vtjnash]

I've seen code written in use (e.g. the code deleted by JuliaInterop/JavaCall.jl@62b7c94#diff-8e308bc5fa4175394c000f10db4abd8cL257) that treats Variadic arguments as the same as regular arguments in a ccall. This was valid in x86, but it violates the platform ABI of x86_64 (a somewhat minor deficiency) and completely ignores the platform ABI of Win64 (and is thus a very bad idea).

Currently the syntax only allows one, compile-time, argument type (T1, T2, T3...), but more flexibility may be necessary, although I don't know how I would express it:

1. perhaps just allow more args than ccall parameters, and auto-detect the types of the remaining parameters?
2. similar idea, but the remaining parameters must be tuples of (Type, value)?
3. ???

[vtjnash]

see #6661 julep

[vtjnash]

@ccall now implements this