test:add struct size test

cl/_testrt/structsize/in.go

@@ -0,0 +1,19 @@
+package main
+
+import (
+	"unsafe"
+
+	"github.com/goplus/llgo/c"
+)
+
+type Foo struct {
+	A byte
+	B uint8
+	C uint16
+	D byte
+	E [8]int8
+}
+
+func main() {
+	c.Printf(c.Str("%d"), unsafe.Sizeof(Foo{}))
+}