The existing implementation produces correct results with a wide range of inputs,
but invalid results asymptotically. With this change we ensure correct asymptotic results
on s390x

Fixes #26477

Change-Id: I760c1f8177f7cab2d7622ab9a926dfb1f8113b49
Reviewed-on: https://go-review.googlesource.com/127119Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>