cmd/addr2line: exit 0 for --help
cmd/addr2line: exit 0 for --help

This is what pprof expects, or else it won't use the program.
And if it doesn't use the program, it gets very bad results.

Fixes issue 4818.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7728043
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go