dashboard: add netbsd-arm-bsiegert
dashboard: add netbsd-arm-bsiegert

This seems to be the last builder still using the old builder
binary and not buildlet. It needs an entry in here to switch
to buildlet and stage0.

Change-Id: If2fdd2298ab88d56b6850061050caf27ca7c8983
Reviewed-on: https://go-review.googlesource.com/c/build/+/168061
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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