dashboard: add FreeBSD arm64 builder
dashboard: add FreeBSD arm64 builder

Updates golang/go#24715

Change-Id: I8eb3f6372d69718b6f453e5415cdf16aa00ccae6
Reviewed-on: https://go-review.googlesource.com/c/build/+/198597
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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