runtime: fix SIGILL in checkvectorfacility on s390x
runtime: fix SIGILL in checkvectorfacility on s390x

STFLE does not necessarily write to all the double-words that are
requested. It is therefore necessary to clear the target memory
before calling STFLE in order to ensure that the facility list does
not contain false positives.

Fixes #17032.

Change-Id: I7bec9ade7103e747b72f08562fe57e6f091bd89f
Reviewed-on: https://go-review.googlesource.com/28850
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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