cmd/coordinator: update the count for amd64 Mac Hosts
cmd/coordinator: update the count for amd64 Mac Hosts

This change updates the number of Mac hosts that are expected to
connect to the coordinator. There is now a distinction between there
being a amd64 and arm64 Mac hosts. We have reduced the count of amd64
hosts to 8 and have been adding arm64 Macs.

Updates golang/go#45088

Change-Id: I97de966c8398ceb0e5cb31d6bbe688de69efc4a5
Reviewed-on: https://go-review.googlesource.com/c/build/+/307731
Trust: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
Run-TryBot: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build