math: add tests for erf and erfc

Test large but not infinite arguments.

This CL adds a test which breaks s390x.  Don't submit until
a fix for that is figured out.

Update #26477

Change-Id: Ic86739fe3554e87d7f8e15482875c198fcf1d59c
Reviewed-on: https://go-review.googlesource.com/125641
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/math/all_test.go

@@ -946,6 +946,8 @@ var vferfSC = []float64{
 	0,
 	Inf(1),
 	NaN(),
+	-1000,
+	1000,
 }
 var erfSC = []float64{
 	-1,
@@ -953,17 +955,23 @@ var erfSC = []float64{
 	0,
 	1,
 	NaN(),
+	-1,
+	1,
 }
 
 var vferfcSC = []float64{
 	Inf(-1),
 	Inf(1),
 	NaN(),
+	-1000,
+	1000,
 }
 var erfcSC = []float64{
 	2,
 	0,
 	NaN(),
+	2,
+	0,
 }
 
 var vferfinvSC = []float64{