main.star: add openbsd-ppc64, linux-riscv64, freebsd-riscv64 builders

Since the list of BUILDER_TYPES is nearly sorted, keep that up,
and sort (using 'Sort Lines' in $EDITOR) two of Linux run mods.

For golang/go#63480.
For golang/go#63481.
For golang/go#63482.

Change-Id: Icef633ab7a0d53b5807c2ab4a076d74c291dc0ea
Reviewed-on: https://go-review.googlesource.com/c/build/+/534976
TryBot-Bypass: Dmitri Shuralyov <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>

Author: dmitshur