cmd/dist: more build fixes
cmd/dist: more build fixes

Flush stdout before writing to stderr, to avoid
reordering output.

Allow amd64 from uname -m (FreeBSD).

TBR=golang-dev
CC=golang-dev
http://codereview.appspot.com/5629051
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go