build: fix a mixup with the cmake variables

This adjusts the windows build for cross-compiling to use the correct
variable.  Import libraries will be generated and tracked properly if
the CMAKE_SYSTEM_NAME is set to Windows.  It does not matter what the
build system is.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -1312,7 +1312,7 @@ function(_add_swift_library_single target name)
   # doing so will result in incorrect symbol resolution and linkage.  We created
   # import library targets when the library was added.  Use that to adjust the
   # link libraries.
-  if(SWIFTLIB_SINGLE_SDK STREQUAL WINDOWS AND NOT CMAKE_HOST_SYSTEM_NAME STREQUAL Windows)
+  if(SWIFTLIB_SINGLE_SDK STREQUAL WINDOWS AND NOT CMAKE_SYSTEM_NAME STREQUAL Windows)
     foreach(library_list LINK_LIBRARIES INTERFACE_LINK_LIBRARIES PRIVATE_LINK_LIBRARIES)
       set(import_libraries)
       foreach(library ${SWIFTLIB_SINGLE_${library_list}})