As suggested by comments from the review of CL 168478, this adds
Go code to do reverse bytes and removes the asm code, as well
as making a few cosmetic changes.

Change-Id: I08276a11222e03c3b42f4c9dc0d10a371a418be7
Reviewed-on: https://go-review.googlesource.com/c/go/+/203937
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <mike.munday@ibm.com>