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.

src/runtime/string.go

@@ -166,9 +166,10 @@ func decodeUTF8(s string, index uintptr) (rune, uintptr) {
 	}
 }
 
-// indexByte returns the index of the first instance of c in s, or -1 if c is not present in s.
-//go:linkname indexByte strings.IndexByte
-func indexByte(s string, c byte) int {
+// indexByteString returns the index of the first instance of c in s, or -1 if c
+// is not present in s.
+//go:linkname indexByteString internal/bytealg.IndexByteString
+func indexByteString(s string, c byte) int {
 	for i := 0; i < len(s); i++ {
 		if s[i] == c {
 			return i