libgo: update to Go1.16.3 release Change-Id: I5f73fa30d5e5865f59b7eb00204c6e5d7661fe21 Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/309490 Trust: Ian Lance Taylor <iant@golang.org> Reviewed-by: Cherry Zhang <cherryyz@google.com> Reviewed-by: Than McIntosh <thanm@google.com>