encoding/base64: use internal/byteorder

encoding/base64: use internal/byteorder

This permits us to remove the dependency on reflect.

Change-Id: I60b1e9fd713f340bfd5eec2edfa58fc724a8e2d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/641936
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Bypass: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>