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

pombredanne/go-deleteme,gnanderson/go,tv42/old-go,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,dsriram/golang,dsriram/golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,dsriram/golang,gnanderson/go,dsriram/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,tv42/old-go