gc: allow colon in //line file name
gc: allow colon in //line file name

Assume last colon introduces line number.

Fixes issue 2543.

R=ken2
CC=golang-dev
http://codereview.appspot.com/5485047

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go