Support Windows ARM64 builds.

[hjyamauchi]

Handle the "ARM64" system processor type.

Checklist

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

[hjyamauchi]

@compnerd

[compnerd]

@swift-ci please test

[hjyamauchi]

@compnerd Can this be merged?

[compnerd]

Yeah, I suppose that this should also go into release/1.1. I hope not too many more - Windows, Darwin: ARM64; Linux: aarch64. The case insensitivity is annoying, but c'est la vie.

[lorentey]

It turns out we already have this change on main, release/1.1 and release/1.0! -- GitHub didn't flag the PR as invalid because it duplicates a previous change exactly, so it can technically be merged without any conflicts. 😮

The root of the issue is that swift's release/5.10 branch is picking up an obsolete tag of this package. (The PR that updated the branch specs only landed on swift's main branch, not release/5.10.) We may be too late to retry landing that for 5.10.0, but perhaps it could be possible to do it in a 5.10.1 update.

What a mess!