math/big: fix s390x test build tags
math/big: fix s390x test build tags

The tests failed to compile when using the math_big_pure_go tag on
s390x.

Change-Id: I2a09f53ff6562ab9bc9b886cffc0f6205bbfcfbb
Reviewed-on: https://go-review.googlesource.com/36956
Run-TryBot: Michael Munday <8f145917441a88646319703b14b4bef730f48765@ca.ibm.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

ImJasonH/go,linux-on-ibm-z/go,akutz/go,golang/go,polyverse-security/go,Cofyc/go,josharian/go,codestation/go,ImJasonH/go,derekmarcotte/go,linux-on-ibm-z/go,vsekhar/elastic-go,vsekhar/elastic-go,akutz/go,ImJasonH/go,derekmarcotte/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,polyverse-security/go,christopher-henderson/Go,Cofyc/go,derekmarcotte/go,nawawi/go,CAFxX/go,derekmarcotte/go,akutz/go,derekmarcotte/go,CAFxX/go,golang/go,Cofyc/go,Cofyc/go,Cofyc/go,codestation/go,golang/go,josharian/go,nawawi/go,vsekhar/elastic-go,codestation/go,josharian/go,vsekhar/elastic-go,akutz/go,cloudflare/go,cloudflare/go,Cofyc/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,josharian/go,akutz/go,polyverse-security/go,linux-on-ibm-z/go,polyverse-security/go,Cofyc/go,akutz/go,linux-on-ibm-z/go,Cofyc/go,vsekhar/elastic-go,CAFxX/go,CAFxX/go,josharian/go,cloudflare/go,derekmarcotte/go,codestation/go,ImJasonH/go,akutz/go,ImJasonH/go,codestation/go,golang/go,golang/go,derekmarcotte/go,vsekhar/elastic-go,CAFxX/go,linux-on-ibm-z/go,josharian/go,nawawi/go,golang/go,nawawi/go,vsekhar/elastic-go,polyverse-security/go,linux-on-ibm-z/go,ImJasonH/go,ImJasonH/go,nawawi/go,golang/go,CAFxX/go,linux-on-ibm-z/go,codestation/go,polyverse-security/go,derekmarcotte/go,polyverse-security/go,vsekhar/elastic-go,akutz/go,polyverse-security/go,CAFxX/go,josharian/go,codestation/go,ImJasonH/go,nawawi/go,linux-on-ibm-z/go,codestation/go,josharian/go,cloudflare/go