libgo: update to Go1.13

Change-Id: I0f0d8f798ec2de82008694eda8afbabaf9fdc0c0
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/194698
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>