Add support for Apple Silicon in SwiftSupport.cmake (#257)
Add support for Apple Silicon in SwiftSupport.cmake (#257)

This matches the behavior of [a similar `elseif` branch in the Swift toolchain itself](https://github.com/apple/swift/blob/b80e4bf8f866be26a4108bd92f97b2a182662642/CMakeLists.txt#L594).
apple/swift-argument-parser,apple/swift-argument-parser