x/build: add LUCI freebsd-riscv64 builder

[mengzhuo]

Following the instructions at Dashboard builders:

hostname freebsd-riscv64-mengzhuo

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

[gopherbot]

Change https://go.dev/cl/534976 mentions this issue: main.star: add openbsd-ppc64, linux-riscv64, freebsd-riscv64 builders

[dmitshur]

Thanks. Here's the resulting certificate: freebsd-riscv64-mengzhuo-1697128441.cert.txt.

I've mailed CLs to define your new builder in LUCI and will comment once that's done.

[dmitshur]

Step 3 is complete (CL 534976 is submitted), so you should be able to proceed with the next steps. Please feel free to comment here if you see something unexpected, or run into a problem that the documentation doesn't cover. Thanks.

[cagedmantis]

cc @golang/release

[dmitshur]

Just checking in here, how's it going with steps 4-6 for this builder @mengzhuo?