s390x doesn't have sendfile64 so apply the same fix as MIPS
(eebf7d27) and just use sendfile.

Change-Id: If8fe2e974ed44a9883282430157c3545d5bd04bd
Reviewed-on: https://go-review.googlesource.com/21892Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>