llgo/ssa.SetRuntime: prevent multiple loading of runtime

cl/compile_test.go

@@ -20,6 +20,7 @@ import (
 	"testing"
 
 	"github.com/goplus/llgo/cl/cltest"
+	"github.com/goplus/llgo/ssa"
 )
 
 func testCompile(t *testing.T, src, expected string) {
@@ -36,7 +37,7 @@ func TestFromTestdata(t *testing.T) {
 }
 
 func TestRuntime(t *testing.T) {
-	cltest.Pkg(t, "github.com/goplus/llgo/internal/runtime", "../internal/runtime/llgo_autogen.ll")
+	cltest.Pkg(t, ssa.PkgRuntime, "../internal/runtime/llgo_autogen.ll")
 }
 
 func TestVar(t *testing.T) {