cmd/addr2line: accept optional "0x" prefix for addresses.
cmd/addr2line: accept optional "0x" prefix for addresses.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/91250043

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang