Experimental varargs fix

As the title says, this is an **experimental** fix for the varargs horror (see https://github.com/ldc-developers/ldc/issues/702, https://github.com/ldc-developers/ldc/issues/172, https://github.com/ldc-developers/ldc/issues/73 and https://github.com/ldc-developers/druntime/pull/14).
It seems to be working fine on Win64 in combination with the ```core.vararg``` and ```core.stdc.stdarg``` patches from https://github.com/ldc-developers/druntime/pull/14.
**TODO**: these 2 files will definitely need to be reworked for other platforms too (core.vararg simply forwarding to core.stdc.stdarg and the latter completely reworked/simplified, especially for non-Windows x86_64!). That shouldn't be too tricky and will probably yield something like https://github.com/kinke/druntime/commit/7bd091f4dd31f32c204951f5605e879bf00b3074. So @klickverbot @redstar etc.: please experiment on Linux/OSX/PPC etc.
In particular, it probably gets 3 and 5-7-byte structs wrong.