go/build: enable cgo on freebsd/arm

Now that it works we need to turn it back on.

Fixes #10119.

Change-Id: I9c62d3026f7bb62c49a601ad73f33bf655372915
Reviewed-on: https://go-review.googlesource.com/13162Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/build/build.go

@@ -265,6 +265,7 @@ var cgoEnabled = map[string]bool{
 	"dragonfly/amd64": true,
 	"freebsd/386":     true,
 	"freebsd/amd64":   true,
+	"freebsd/arm":     true,
 	"linux/386":       true,
 	"linux/amd64":     true,
 	"linux/arm":       true,