math: correct Atan2(±y,+∞) = ±0 on s390x

The s390x assembly implementation was previously only handling this
case correctly for x = -Pi.  Update the special case handling for
any y.

Fixes #35446

Change-Id: I355575e9ec8c7ce8bd9db10d74f42a22f39a2f38
Reviewed-on: https://go-review.googlesource.com/c/go/+/223420


Run-TryBot: Brian Kessler <brian.m.kessler@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <mike.munday@ibm.com>
Reviewed-by: Robert Griesemer <gri@golang.org>