Determine how to build aspnetcore's hosting bundle

[mmitche]

ASPNetCore's hosting bundle requires some x86 and x64 artifacts to build. If we went with a relatively strict set of verticals, we would not build the x86 and x64 bits on the same machine. That means the hosting bundle will need to be built in the join. Questions that need answering:

• How is it built at the VMR join?
• How is it still built at repo level for validation purposes? How do we sink the join lower in the stack for repo-level validation?

T-Shirt size: Medium This is part of the other join point item.

[wtgodbe]

2 or 3 years ago, I investigated doing parallel x64/x86 builds w/ a join job at the end: #17599. It turned out to actually be slower, because the time we saved by parallelizing was lost thru artifact upload/downloads, and machine provisioning. It did work, though, so if this is just about fitting VMR build requirements & we don't mind losing a few minutes of build time, this is definitely doable.