The input buffer is aligned to a doubleword boundary to
improve performance of the vector instructions. The pure
Go implementation is used to align the input data, and is
also used when the vector instructions are not available
or the data length is less than 64 bytes.

Change-Id: Ie259a5f2f1562bcc17961c99e5776c99091d6bed
Reviewed-on: https://go-review.googlesource.com/22201Reviewed-by: Michael Munday <munday@ca.ibm.com>
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bill O'Farrell <billotosyr@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>