cmd/coordinator, internal/buildgo: add RunMake

This moves RunMake from coordinator.go, allowing other packages to
build Go on a buildlet.

Updates golang/go#19871

Change-Id: Ic0e7c056020ef434f0620ae816f52b9112ea1c8d
Reviewed-on: https://go-review.googlesource.com/44176
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
3 files changed