cmd/buildlet/stage0: also write to stderr when using windows serial port
cmd/buildlet/stage0: also write to stderr when using windows serial port

Change-Id: Icd6d091ad896ad9f27e41c7287592569be634b7f
Reviewed-on: https://go-review.googlesource.com/51335
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build