mips64 strace doesn't support sendfile64 and will error out if we
specify that with `-e trace='. So we use sendfile for mips64 here.

Change-Id: If5e2bb39866ca3a77dcc40e4db338ba486921d89
Reviewed-on: https://go-review.googlesource.com/14455Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>