cmd/gc: put 'not used' message on correct line

Fixes issue 4663.

R=ken2
CC=golang-dev
https://codereview.appspot.com/7235081
cmd/gc: silence redundant error prints for misuse of [...]int

Fixes issue 4452.

R=ken2
CC=golang-dev
https://codereview.appspot.com/7241065