Ceil and Trunc of -0.2 return -0, not +0, but we didn't test that.

Updates #23647

Change-Id: Idbd4699376abfb4ca93f16c73c114d610d86a9f2
Reviewed-on: https://go-review.googlesource.com/91335
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>