x/build: add LUCI openbsd-riscv64 builder

[mengzhuo]

Following the instructions at Dashboard builders:

hostname openbsd-riscv64-mengzhuo

CSR is openbsd-riscv64-mengzhuo.csr.txt
since Github doesn't seem to allow attaching with the name openbsd-riscv64-mengzhuo.csr origin issue asked for.

Builder info(uname -a):
OpenBSD mars-1.lan 7.4 GENERIC.MP#427 riscv64

cc @4a6f656c

[gopherbot]

Change https://go.dev/cl/545536 mentions this issue: main.star: add netbsd-arm, netbsd-arm64, openbsd-riscv64 builders

[dmitshur]

Here's the resulting certificate: openbsd-riscv64-mengzhuo-1701367184.cert.txt.

The builder definitions have been added in CL 545536 so your bot should be able to connect once you follow the rest of the steps on your end.

We have some more work to do to make the dependencies built for the openbsd/riscv64 port and available in CIPD, which will be needed for the builds to complete successfully. We'll update this issue once that's done.

Edit: Note that because the openbsd/riscv64 port is a new port in the process of being added (#55999), rather than a port that exists in a released version of Go, we have to do additional work to be able to build its dependencies, so it will take longer.