Swift SDKs Cross Compilation Support

[owenv]

In order to achieve feature parity with the current SwiftPM build system, we need to support cross-compilation using Swift SDKs. Based on initial experimentation it looks like we can bridge a Swift SDK to Swift Build's internal SDK and Toolchain abstraction to handle both tool discovery and selecting the right sysroot, etc.

[jakepetroules]

Evolution proposal for Swift SDKs: https://github.com/swiftlang/swift-evolution/blob/main/proposals/0387-cross-compilation-destinations.md