runtime: move panicindex/panicslice to Go.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/130210043

src/pkg/runtime/panic.c

@@ -459,18 +459,6 @@ runtime·dopanic(int32 unused)
 	runtime·exit(2);
 }
 
-void
-runtime·panicindex(void)
-{
-	runtime·panicstring("index out of range");
-}
-
-void
-runtime·panicslice(void)
-{
-	runtime·panicstring("slice bounds out of range");
-}
-
 void
 runtime·throwreturn(void)
 {

src/pkg/runtime/panic.go

+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime
+
+func panicindex() {
+	panic(errorString("index out of range"))
+}
+
+func panicslice() {
+	panic(errorString("slice bounds out of range"))
+}