libgo: update to Go1.16 release

Change-Id: Id51ade2ee4c9c036489e86b4c48cbef9d00bef24
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/293793
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>