add math/rand (#403)

c/math/rand

_demo/rand/rand.go

@@ -0,0 +1,15 @@
+package main
+
+import (
+	"github.com/goplus/llgo/c"
+	"github.com/goplus/llgo/c/math/rand"
+)
+
+func main() {
+	var s c.Uint = 6
+	rand.Srand(s)
+	rr := rand.RandR(&s)
+	r := rand.Rand()
+	println("r:", r)
+	println("rr:", rr)
+}