syscall: fix FreeBSD 386 sendfile
The previous version was not handling an off_t (64 bit)
parameter on 32 bit i386 systems.  This patch splits sendfile
into two implementations in their respective arch-specific files.
Tested on FreeBSD amd64 and i386.

R=bradfitz
CC=golang-dev
https://golang.org/cl/6356048
3 files changed