0.15.1 used stdcall convention everywhere. But the ABI change in 0.15.2
requires use of the C calling convention for variadic calls on x86
because the stdcall convention does not support vararg functions.
Solution is to check the llvm::FunctionType if the function has varargs.
This commit also makes the C calling convention the global default
because according to the LLVM documentation the fastcc convention used
for D linkage does not support varargs.
This fixes issue #1000.
