bytes, strings: update s390x code to match amd64 changes

Updates the s390x-specific files in these packages with the changes
to the amd64-specific files made during the review of CL 31690. I'd
like to keep these files in sync unless there is a reason to
diverge.

Change-Id: Id83e5ce11a45f877bdcc991d02b14416d1a2d8d2
Reviewed-on: https://go-review.googlesource.com/32574


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>