libgo: update to Go1.16beta1 release

This does not yet include support for the //go:embed directive added
in this release.

Change-Id: I5cb78e4d6b6c4780e0781e88d1f5072225767841
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/280172
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
1185 files changed