MacPorts release for macOS 11 Big Sur and Apple Silicon

[Ryan Schmidt]

Apple has announced that macOS 11 Big Sur will be released tomorrow, November 12, 2020, and that Apple Silicon machines will be available starting November 17.

We will need a new release of MacPorts (2.6.4 or 2.7.0) to support Apple Silicon. What we have in git master is probably most of the way there already, but I'd like to get in a fix for this issue before we release:

https://trac.macports.org/ticket/61430

As for macOS 11 on x86_64, I'm not sure if we need a new MacPorts release for that already or whether we can just compile 2.6.3 for macOS 11. We probably at least need to list macOS 11 and Xcode 12 as supported:

https://github.com/macports/macports-base/pull/194
https://lists.macports.org/pipermail/macports-dev/2020-November/042563.html

Are there any other fixes that need to go into the next release that haven't already been committed? Any PRs not yet merged that we want?