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>