Add support for Android

[grynspan]

Add support for Android:

• Audit our platform-specific code and add branches for Android (mostly, but not entirely, the same as Linux)
• Set up team workstations or VMs
• Set up CI jobs

[finagolfin]

Note that while cross-compiling some proportion of Testing tests has worked for a while now, long-standing SwiftPM bugs like swiftlang/swift-package-manager#8094 have broken cross-compiling this repo's tests to any platform, so the remaining steps here are blocked on those underlying bugs, which I'm told the new swift-build backend may soon fix. Instead, I have been occasionally natively building the trunk toolchain on Android in the Termux app then testing this repo, and with #1373 or an earlier patch, all tests passed.