doc/go1.5: mention cgo is supported on linux/arm64 (ext. link only)

Change-Id: I1dcca264d9cc900aad2d7737073cc01fe574bf55
Reviewed-on: https://go-review.googlesource.com/8623Reviewed-by: Minux Ma <minux@golang.org>

doc/go1.5.txt

@@ -10,7 +10,7 @@ Go 1.4 required to build (https://golang.org/cl/2470, https://golang.org/cl/2993
 
 New Ports:
 darwin/arm, a.k.a iOS. (https://golang.org/cl/2118, 2119, 3273, 2121, 2122, ..., 2127)
-linux/arm64
+linux/arm64 (cgo is supported, but only with external linking)
 
 Removed Ports:
 dragonfly/386 (https://golang.org/cl/7543)