cmd/ld: always do external link for -linkmode=external
cmd/ld: always do external link for -linkmode=external

There are tests in run.bash for -linkmode=external.

Fixes issue 5238.

R=golang-dev, bradfitz, remyoudompheng, r
CC=golang-dev
https://codereview.appspot.com/8716044

wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang