cmd/internal/objfile: fix dissassembly of Plan 9 object files

This is a reapplication of CL 93520045 (changeset 5012df7fac58)
since that was lost during the move to an internal package.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/134020043
runtime: fix Plan 9 build for new C calling convention

LGTM=0intro, rsc
R=rsc, 0intro
CC=golang-codereviews
https://codereview.appspot.com/132320043