Investigate parallelizing x64/x86 build job

[wtgodbe]

The x64/x86 leg of our CI builds is currently the long pole. It builds x64, then x86, then some components that require both. We should investigate if it's possible to have x64 and x86 run concurrently in separate legs, then have a 3rd leg which restores the artifacts of both, and proceeds with the build of the installers & whatnot.

CC @aspnet/build @rynowak

[rynowak]

Do we really need to go that far? I suppose I was wondering if there's redundant work between the x64 and x86 steps, ie: building all of the managed code twice. I think it would be helpful to get a picture of what the current stuff does.

[ghost]

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[wtgodbe]

@dougbu assigning this to you as it's similar to what you're doing w/ removing redundant build jobs