cmd/ld: fix "_image_base__ not defined" problem for cmd/8l. Fixes issue 6431.
cmd/ld: fix "_image_base__ not defined" problem for cmd/8l.
Fixes issue 6431.

Change suggested by kin.wilson.za.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/13314048

apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go