cgo: enable external linking mode on FreeBSD amd64.
cgo: enable external linking mode on FreeBSD amd64.

Tested on FreeBSD 9.1 amd64, per rsc's instructions at
https://groups.google.com/d/topic/golang-dev/HjRTLvRsJXo/discussion .

R=golang-dev, lucio.dere, devon.odell, rsc
CC=golang-dev
https://codereview.appspot.com/7664044

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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