main.star: start adding misc-compile builders

Misc-compile builders are going to be TryBots (i.e., builders that run
before CL submission) that perform compile-only testing across all non-
first-class ports. This is viable to do on highly available machines
thanks to cross-compilation, relying on sharding across multiple such
machines to get it as fast as we need it to be.

Only postsubmit to begin with to avoid disturbing anyone while this
rolls out. Once it works I will add presubmit and stop postsubmit.

For golang/go#61698.

Change-Id: I73b9d2a56c66196469812161251b6dd9aea554db
Reviewed-on: https://go-review.googlesource.com/c/build/+/515035
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Bypass: Dmitri Shuralyov <[email protected]>

Author: dmitshur