runtime: only implement CountString for required platforms

testdata/stdlib.go

@@ -18,4 +18,5 @@ func main() {
 
 	// package strings
 	fmt.Println("strings.IndexByte:", strings.IndexByte("asdf", 'd'))
+	fmt.Println("strings.Replace:", strings.Replace("An example string", " ", "-", -1))
 }

testdata/stdlib.txt

@@ -3,3 +3,4 @@ stdout: 1
 stderr: 2
 pseudorandom number: 1298498081
 strings.IndexByte: 2
+strings.Replace: An-example-string