runtime: preallocate panic errors for index and slice

This avoids allocating at the panic sites.

LGTM=r, khr
R=golang-codereviews, r, khr
CC=dvyukov, golang-codereviews, iant, khr
https://golang.org/cl/136020043

src/pkg/runtime/panic.go

@@ -4,10 +4,14 @@
 
 package runtime
 
+var indexError = error(errorString("index out of range"))
+
 func panicindex() {
-	panic(errorString("index out of range"))
+	panic(indexError)
 }
 
+var sliceError = error(errorString("slice bounds out of range"))
+
 func panicslice() {
-	panic(errorString("slice bounds out of range"))
+	panic(sliceError)
 }