cmd/gc: insert \n at end of every input file

Not just the first one.

Fixes issue 5433.
Fixes issue 5913.

R=ken2
CC=golang-dev
https://codereview.appspot.com/12028049
cmd/gc: don't report mark if other things are broken

Fixes issue 5598.

R=ken2
CC=golang-dev
https://codereview.appspot.com/12104043