main.star: add linux-ppc64le_power{9,10} builders

For golang/go#66667.
For golang/go#64660.

Change-Id: If36ab865d6b6250b27b2c4d2359603ea839ceadf
Reviewed-on: https://go-review.googlesource.com/c/build/+/576138
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur