loader/cgo: add support for function pointers

testdata/cgo/main.c

@@ -10,6 +10,10 @@ int add(int a, int b) {
 	return a + b;
 }
 
+int doCallback(int a, int b, binop_t callback) {
+	return callback(a, b);
+}
+
 void store(int value, int *ptr) {
 	*ptr = value;
 }

testdata/cgo/main.go

@@ -3,6 +3,7 @@ package main
 /*
 int fortytwo(void);
 #include "main.h"
+int mul(int, int);
 */
 import "C"
 
@@ -23,4 +24,13 @@ func main() {
 	println("15:", *ptr)
 	C.store(25, &n)
 	println("25:", *ptr)
+	cb := C.binop_t(C.add)
+	println("callback 1:", C.doCallback(20, 30, cb))
+	cb = C.binop_t(C.mul)
+	println("callback 2:", C.doCallback(20, 30, cb))
+}
+
+//export mul
+func mul(a, b C.int) C.int {
+	return a * b
 }

testdata/cgo/main.h

@@ -1,5 +1,7 @@
 typedef short myint;
 int add(int a, int b);
+typedef int (*binop_t) (int, int);
+int doCallback(int a, int b, binop_t cb);
 typedef int * intPointer;
 extern int global;
 void store(int value, int *ptr);

testdata/cgo/out.txt

@@ -6,3 +6,5 @@ longlong: -1099511627776
 global: 3
 15: 15
 25: 25
+callback 1: 50
+callback 2: 600