[release-branch.go1.7] runtime: fix SIGILL in checkvectorfacility on s390x

[release-branch.go1.7] 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 <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/31267
Reviewed-by: Michael Munday <munday@ca.ibm.com>