go/build: cgo is supported on Linux/ARM

        Last Part of CL 5601044 (cgo: Linux/ARM support)
        Fixes #1880.

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/5988057

src/pkg/go/build/build.go

@@ -215,6 +215,7 @@ var cgoEnabled = map[string]bool{
 	"darwin/amd64":  true,
 	"linux/386":     true,
 	"linux/amd64":   true,
+	"linux/arm":     true,
 	"freebsd/386":   true,
 	"freebsd/amd64": true,
 	"windows/386":   true,