cmd/{addr2line,objdump}: fix finding pclntab and symtab for pe and plan9obj

Broken by 8b5fc7c59d05.

Update issue 8092

LGTM=iant, alex.brainman
R=rsc, iant, alex.brainman
CC=golang-codereviews
https://codereview.appspot.com/138770043

Committer: Ian Lance Taylor <iant@golang.org>