The runtime and stubs are built for ALL targets, not specific ones. This allows
us to configure when cross-compiling to Windows again. Collapse the dual
addition of the swiftRuntime into a single build. This unifies the runtime
build for the apple and non-Apple SDKs. The difference here was the ObjC
interop sources. In order to deal with that unification add a CPP macro to
indicate whether the interop sources should be included or not.