Use tarm/serial to direct logs to COM1 on GCE.
Tested with Windows Server 2016.
tarm/serial vendored at 37be519d49878d0098e34fb614136b81e6c87861
Change-Id: I66908a7d4dd0b0c40008a2e7945fc530af4cb1ee
Reviewed-on: https://go-review.googlesource.com/42141
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Jeff Johnson