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://golang.org/cl/7664044

src/cmd/6l/obj.c

@@ -141,6 +141,7 @@ main(int argc, char *argv[])
 		default:
 			sysfatal("cannot use -hostobj with -H %s", headstr(HEADTYPE));
 		case Hlinux:
+		case Hfreebsd:
 			break;
 		}
 	}