main.star: add linux-386-clang15 and linux-arm64-boringcrypto builders

Add these to replace the old coordinator equivalents. Set a known issue
because I don't know if they'll pass right away or need a bit more work.

Change-Id: Idefe2aa1d836c035b57b9a2e7e0e401637b744db
Reviewed-on: https://go-review.googlesource.com/c/build/+/578255
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
5 files changed