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/main.c

@@ -34,6 +34,14 @@ int doCallback(int a, int b, binop_t callback) {
 	return callback(a, b);
 }
 
+int variadic0() {
+	return 1;
+}
+
+int variadic2(int x, int y, ...) {
+	return x * y;
+}
+
 void store(int value, int *ptr) {
 	*ptr = value;
 }

testdata/cgo/main.go

@@ -36,6 +36,10 @@ func main() {
 	cb = C.binop_t(C.mul)
 	println("callback 2:", C.doCallback(20, 30, cb))
 
+	// variadic functions
+	println("variadic0:", C.variadic0())
+	println("variadic2:", C.variadic2(3, 5))
+
 	// equivalent types
 	var goInt8 int8 = 5
 	var _ C.int8_t = goInt8

testdata/cgo/main.h

@@ -10,6 +10,9 @@ int doCallback(int a, int b, binop_t cb);
 typedef int * intPointer;
 void store(int value, int *ptr);
 
+int variadic0();
+int variadic2(int x, int y, ...);
+
 # define CONST_INT 5
 # define CONST_INT2 5llu
 # define CONST_FLOAT 5.8

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