cgo: fix calling CGo callback inside generic function

The package of the generic function might not be set, so we have to call
Origin() for it.

Found while porting mcufont to TinyGo.

testdata/cgo/main.go

@@ -43,6 +43,7 @@ func main() {
 	println("callback 1:", C.doCallback(20, 30, cb))
 	cb = C.binop_t(C.mul)
 	println("callback 2:", C.doCallback(20, 30, cb))
+	genericCallbackCall[int]()
 
 	// variadic functions
 	println("variadic0:", C.variadic0())
@@ -197,3 +198,11 @@ func printBitfield(bitfield *C.bitfield_t) {
 	println("bitfield d:", bitfield.d)
 	println("bitfield e:", bitfield.e)
 }
+
+type Int interface {
+	int | uint
+}
+
+func genericCallbackCall[T Int]() {
+	println("callback inside generic function:", C.doCallback(20, 30, C.binop_t(C.add)))
+}

testdata/cgo/out.txt

@@ -13,6 +13,7 @@ defined expr: 9
 25: 25
 callback 1: 50
 callback 2: 600
+callback inside generic function: 50
 variadic0: 1
 variadic2: 15
 headerfunc: 6