math: add tests for erf and erfc
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 <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go,josharian/go,golang/go,josharian/go,CAFxX/go,golang/go,CAFxX/go,akutz/go,nawawi/go,golang/go,cloudflare/go,akutz/go,cloudflare/go,akutz/go,josharian/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,josharian/go,CAFxX/go,akutz/go,golang/go,cloudflare/go,josharian/go,cloudflare/go,josharian/go,akutz/go,cloudflare/go,akutz/go,akutz/go,nawawi/go,nawawi/go,golang/go,josharian/go,josharian/go,cloudflare/go,CAFxX/go,cloudflare/go,akutz/go