misc/dist: remove exp and old before building
misc/dist: remove exp and old before building

Fixes issue 3317.

R=golang-dev, dsymonds
CC=golang-dev
http://codereview.appspot.com/5784074
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go