go/build: cgo is supported on Linux/ARM         Last Part of CL 5601044 (cgo: Linux/ARM support)         Fixes issue 1880.
go/build: cgo is supported on Linux/ARM
        Last Part of CL 5601044 (cgo: Linux/ARM support)
        Fixes issue 1880.

R=golang-dev, dave, rsc
CC=golang-dev
http://codereview.appspot.com/5988057

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