libgo: update to Go1.16rc1

Change-Id: Icd267f1ac03565ef9e6521eac3434650d788d01a
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/287493
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
186 files changed