🍒 1.2: build: Install libraries in an `arch` sub-folder

[Steelskin]

This is the proper installation scheme for Swift libraries on Windows and prevents having to manually copy them in build.ps1.

Cherry-pick #505

Checklist

• [X] I've read the Contribution Guidelines
• [X] My contributions are licensed under the Swift license.
• [X] I've followed the coding style of the rest of the project.
• [X] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [X] I've added benchmarks covering new functionality (if appropriate).
• [X] I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• [X] I've updated the documentation if necessary.

[Steelskin]

@swift-ci please test

[Steelskin]

@lorentey Thanks for the approval but it looks like I don't have the rights to run the remaining CI or land this. Could you do it for me please?

[lorentey]

I'm working on setting up CI workflows on this branch. Stayed tuned!