net/http: fix TestLinuxSendfile on s390x

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>

src/net/http/fs_test.go

@@ -978,9 +978,9 @@ func TestLinuxSendfile(t *testing.T) {
 
 	syscalls := "sendfile,sendfile64"
 	switch runtime.GOARCH {
-	case "mips64", "mips64le":
-		// mips64 strace doesn't support sendfile64 and will error out
-		// if we specify that with `-e trace='.
+	case "mips64", "mips64le", "s390x":
+		// strace on the above platforms doesn't support sendfile64
+		// and will error out if we specify that with `-e trace='.
 		syscalls = "sendfile"
 	}