test/fixedbugs: add s390x case to issue11656

An instruction consisting of all 0s causes an illegal instruction
signal on s390x. Since 0s are the default in this test this CL just
makes it explicit.

Change-Id: Id6e060eed1a588f4b10a4e4861709fcd19b434ac
Reviewed-on: https://go-review.googlesource.com/20962Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/issue11656.go

@@ -65,6 +65,8 @@ func f(n int) {
 		binary.BigEndian.PutUint32(ill, 0x00000034) // trap
 	case "mips64le":
 		binary.LittleEndian.PutUint32(ill, 0x00000034) // trap
+	case "s390x":
+		binary.BigEndian.PutUint32(ill, 0) // undefined instruction
 	default:
 		// Just leave it as 0 and hope for the best.
 	}