[dev.cc] liblink: fix warnings on Plan 9

warning: src/liblink/asm9.c:501 set and not used: bflag
warning: src/liblink/list9.c:259 format mismatch .5lux INT, arg 4
warning: src/liblink/list9.c:261 format mismatch .5lux INT, arg 3
warning: src/liblink/list9.c:319 more arguments than format VLONG
warning: src/liblink/obj9.c:222 set and not used: autoffset

LGTM=bradfitz, austin
R=rsc, bradfitz
CC=austin, golang-codereviews
https://codereview.appspot.com/175070043