math: fix math.Remainder(-x,x) (for Inf > x > 0)

Modify the |x| == |y| case to return -0 when x < 0.

Fixes #30814.

Change-Id: Ic4cd48001e0e894a12b5b813c6a1ddc3a055610b
Reviewed-on: https://go-review.googlesource.com/c/go/+/167479


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>