[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/28850Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/31267Reviewed-by: Michael Munday <munday@ca.ibm.com>

src/runtime/asm_s390x.s

@@ -20,6 +20,7 @@ TEXT runtime·checkvectorfacility(SB),NOSPLIT,$32-0
 	MOVD    $2, R0
 	MOVD	R1, tmp-32(SP)
 	MOVD    $x-24(SP), R1
+	XC	$24, 0(R1), 0(R1)
 //      STFLE   0(R1)
 	WORD    $0xB2B01000
 	MOVBZ   z-8(SP), R1