libgo: update to Go 1.12.1

Change-Id: I0aa6216b92850d1cd936b75c1cebd65ef4644309
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/167749
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
22 files changed