runtime: use staticbytes in intstring for small v

Triggers 21 times during make.bash.

Change-Id: I7efb34200439256151304bb66cd309913f7c9c9e
Reviewed-on: https://go-review.googlesource.com/110557
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/string.go

@@ -229,8 +229,13 @@ func stringStructOf(sp *string) *stringStruct {
 	return (*stringStruct)(unsafe.Pointer(sp))
 }
 
-func intstring(buf *[4]byte, v int64) string {
-	var s string
+func intstring(buf *[4]byte, v int64) (s string) {
+	if v >= 0 && v < runeSelf {
+		stringStructOf(&s).str = unsafe.Pointer(&staticbytes[v])
+		stringStructOf(&s).len = 1
+		return
+	}
+
 	var b []byte
 	if buf != nil {
 		b = buf[:]