libgo: update to Go1.12rc1

Change-Id: I58f237f8bf828919b7158dd1b71b7335b2cf29b8
Reviewed-on: https://go-review.googlesource.com/c/162881
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
67 files changed