dashboard: add linux-amd64-goamd64v3 builder
dashboard: add linux-amd64-goamd64v3 builder

The purpose of this builder will be to test Go with a non-default
value of the new GOAMD64 environment variable.

For golang/go#48505.
Updates golang/go#45453.

Change-Id: Ic7bf0bd45f3539530ac6540cc3609f87cfdab6f7
Reviewed-on: https://go-review.googlesource.com/c/build/+/386116
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alex Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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