doc/debugging_with_gdb: mention how to disable gc optimization
doc/debugging_with_gdb: mention how to disable gc optimization

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/6353055
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go