main.star: make power{8,9,10} a part of the host, not runmod

Per discussion in go.dev/issue/64660, the intent is to have distinct
hardware test the GOOS=linux GOARCH=ppc64{,le} GOPPC64=power{8,9,10}
Go ports, instead of using the same hardware with a different value
of the GOPPC64 environment variable set.

By now crrev.com/c/5178250 has propagated and the swarming bot reports
in its 'cpu' dimension whether it's a POWER{8,9,10} type of CPU. Update
our configuration accordingly: make it a property of the host which type
of CPU should be matched, and replace the previous power10 runmod in
favor of setting the GOPPC64 env var explicitly as part of the builder
definition (since a runmod is redundant with the host suffix and would
make the builder name more repetitive).

After this change is in, it will become possible to have hardware for
testing the GOOS=linux GOARCH=ppc64le GOPPC64=power10 port connect to
LUCI and not get selected as a host for the LUCI builder now defined
as 'linux-ppc64le_power8' (previously defined as 'linux-ppc64le').

For golang/go#64660.
For golang/go#64588.

Change-Id: I1bf00231c46a29052c3e3dfce92f6bea20fea708
Reviewed-on: https://go-review.googlesource.com/c/build/+/575856
Auto-Submit: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: dmitshur