main.star: add linux-arm64-{longtest,race} builders

The linux/arm64 port is first class and has longtest and race builders
in the old infrastructure. We should have the capacity for it, so port
these builders to LUCI as well.

Change-Id: I3ae2148d2396152a00d9f64421c82c86c9a61edc
Reviewed-on: https://go-review.googlesource.com/c/build/+/574559
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur