Support includes 386, amd64, and arm for both Linux and FreeBSD.
FreeBSD changes courtesy Dave Chapeskie <dchapeskie@gmail.com>

Change-Id: I89dc18533d7178e74ae6c442a2e6272c9bd9692c
Reviewed-on: https://go-review.googlesource.com/8055Reviewed-by: Rob Pike <r@golang.org>