So: update on my efforts. It has become clear that it will be difficult to support armv7 officially within the swiftlang/swift toolchain. And, the only way that makes sense to support it is to build the runtime for the target, then cross-compile from x86_64 or aarch64 to armv7. So, I've launched my effort for this:
Moving forward, this can just be a small community CI and we (as the community) can keep the effort alive of supporting armv7 for future releases.