math: test large negative values as args for trig functions

math: test large negative values as args for trig functions

Sin/Tan are odd, Cos is even, so it is easy to compute the correct
result from the positive argument case.

Change-Id: If851d00fc7f515ece8199cf56d21186ced51e94f
Reviewed-on: https://go-review.googlesource.com/c/go/+/509815
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Srinivas Pokala <Pokala.Srinivas@ibm.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Keith Randall <khr@google.com>