x/build: add LUCI openbsd-arm64 builder

[4a6f656c]

Please add a LUCI builder for openbsd/arm64 with hostname openbsd-arm64-jsing - note that this will be running on the same hardware as the current host-openbsd-arm64-joelsing builder. This machine is not capable of running both forms and will need to be hard migrated at some point in time. CSR for this builder is attached.

openbsd-arm64-jsing.csr.txt

[gopherbot]

Change https://go.dev/cl/584777 mentions this issue: main.star: update openbsd-{arm,arm64,riscv64} builder configurations

[dmitshur]

Here's the certificate: openbsd-arm64-jsing-1715630445.cert.txt.

Please feel free to stop the old coordinator builder on your end whenever it's getting in the way of bringing up the new LUCI builder.

[dmitshur]

@4a6f656c The builder seems to be doing well. It was fully passing at tip and on release-branch.go1.23, and passing short of 2-3 tests on older release branches. In case you're not aware, the bot has died during https://ci.chromium.org/b/8740858297282318161 about 15 hours ago and hasn't come back since then.