cmd/{5,6,8}g: reduce size of Prog and Addr

cmd/{5,6,8}g: reduce size of Prog and Addr

5g: Prog went from 128 bytes to 88 bytes
6g: Prog went from 174 bytes to 144 bytes
8g: Prog went from 124 bytes to 92 bytes

There may be a little more that can be squeezed out of Addr, but alignment will be a factor.

All: remove the unused pun field from Addr

R=rsc, minux.ma
CC=golang-dev
https://golang.org/cl/6922048