compiler,runtime: fix multiple definitions of a single function

strings.IndexByte was implemented in the runtime up to Go 1.11. It is
implemented using a direct call to internal/bytealg.IndexByte since Go
1.12.

Make sure we remain compatible with both.

testdata/stdlib.go

@@ -4,12 +4,18 @@ import (
 	"fmt"
 	"math/rand"
 	"os"
+	"strings"
 )
 
 func main() {
+	// package os, fmt
 	fmt.Println("stdin: ", os.Stdin.Fd())
 	fmt.Println("stdout:", os.Stdout.Fd())
 	fmt.Println("stderr:", os.Stderr.Fd())
 
+	// package math/rand
 	fmt.Println("pseudorandom number:", rand.Int31())
+
+	// package strings
+	fmt.Println("strings.IndexByte:", strings.IndexByte("asdf", 'd'))
 }