cmd/cgo: increase s390x int type size to 8 bytes

The size of the int type in Go on s390x is 8 bytes, not 4.

Change-Id: I1a71ce8c9925f3499abb61c1aa4f6fa2d2ec0d7e
Reviewed-on: https://go-review.googlesource.com/21760Reviewed-by: Bill O'Farrell <billotosyr@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/cgo/main.go

@@ -156,7 +156,7 @@ var intSizeMap = map[string]int64{
 	"ppc64":    8,
 	"ppc64le":  8,
 	"s390":     4,
-	"s390x":    4,
+	"s390x":    8,
 }
 
 var cPrefix string