build: fix unsafe.Sizeof for llgo:type C

cl/_testrt/unsafe/in.go

@@ -0,0 +1,30 @@
+package main
+
+import (
+	"unsafe"
+)
+
+//llgo:type C
+type T func()
+
+type M struct {
+	v  int
+	fn T
+}
+
+type N struct {
+	v  int
+	fn func()
+}
+
+func main() {
+	if unsafe.Sizeof(*(*T)(nil)) != unsafe.Sizeof(0) {
+		panic("error")
+	}
+	if unsafe.Sizeof(*(*M)(nil)) != unsafe.Sizeof([2]int{}) {
+		panic("error")
+	}
+	if unsafe.Sizeof(*(*N)(nil)) != unsafe.Sizeof([3]int{}) {
+		panic("error")
+	}
+}