main.star: add netbsd-arm, netbsd-arm64, openbsd-riscv64 builders

For golang/go#63698.
For golang/go#63614.
For golang/go#64176.

Change-Id: I2a203cd2a1e2e80ee44cfd5ce11c1ce5bbd60002
Reviewed-on: https://go-review.googlesource.com/c/build/+/545536
Reviewed-by: Heschi Kreinick <[email protected]>
TryBot-Bypass: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur