cgo: add support for variadic functions

This doesn't yet add support for actually making use of variadic
functions, but at least allows (unintended) variadic functions like the
following to work:

    void foo();

testdata/cgo/out.txt

@@ -12,6 +12,8 @@ defined char: 99
 25: 25
 callback 1: 50
 callback 2: 600
+variadic0: 1
+variadic2: 15
 bool: true true
 float: +3.100000e+000
 double: +3.200000e+000