dashboard: add linux-arm64-boringcrypto builder
dashboard: add linux-arm64-boringcrypto builder

CL 423362 adds boringcrypto support for linux/arm64.
Add a builder configuration to test that.

Because the builder is on AWS, and because there's
essentially no architecture variation in boringcrypto,
this builder can be left out of the trybot set:
the linux-amd64-boringcrypto trybot will catch
boringcrypto issues, and the linux-arm64-aws trybot
will catch non-boringcrypto arm64 issues.
Any arm64-specific boringcrypto can be dealt with
via slowbot or by finding out on the dashboard
post-submit for now.

Change-Id: Ic257ec02baece554cc178155133706f4d43a7ada
Reviewed-on: https://go-review.googlesource.com/c/build/+/423574
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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