cmd/5l: reduce the size of Prog and Sym

cmd/5l: reduce the size of Prog and Sym

Prog
* Remove the unused Prog* dlink
* note that align is also unused, but removing it does not help due to alignment issues.

Saves 4 bytes, sizeof(Prog): 84 => 80.

Sym
* Align {u,}char fields on word boundaries

Saves 4 bytes, sizeof(Sym): 136 => 132.

Tested on linux/arm and freebsd/arm.

R=minux.ma, remyoudompheng, rsc
CC=golang-dev
https://golang.org/cl/7106050