[release-branch.go1.9] bytes, strings: fix comparison of long byte slices on s390x
    
    The existing implementation of bytes.Compare on s390x doesn't work properly for slices longer
    than 256 elements. This change fixes that. Added tests for long strings and slices of bytes.
    
    Fixes #26118
    
    Change-Id: If6d8b68ee6dbcf99a24f867a1d3438b1f208954f
    Reviewed-on: https://go-review.googlesource.com/121495
    Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
    Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-on: https://go-review.googlesource.com/124137
    Run-TryBot: Ian Lance Taylor <iant@golang.org>
    Reviewed-by: Ian Lance Taylor <iant@golang.org>