build: use older means of getting the swiftrt object

This seems to be failing for some reason.  Fallback to the older way to
get the path to the object.  This should repair the build.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -821,7 +821,15 @@ function(_add_swift_library_single target name)
               ${SWIFTLIB_SINGLE_XCODE_WORKAROUND_SOURCES})
   if("${SWIFT_SDK_${SWIFTLIB_SINGLE_SDK}_OBJECT_FORMAT}" STREQUAL "ELF" AND SWIFTLIB_TARGET_LIBRARY)
     if("${libkind}" STREQUAL "SHARED")
-      target_sources(${target} PRIVATE $<TARGET_OBJECTS:swiftImageRegistrationObject-${SWIFT_SDK_${sdk}_LIB_SUBDIR}-${SWIFTLIB_SINGLE_ARCHITECTURE}>)
+      # TODO(compnerd) switch to the generator expression when cmake is upgraded
+      # to a version which supports it.
+      # target_sources(${target} PRIVATE $<TARGET_OBJECTS:swiftImageRegistrationObject-${SWIFT_SDK_${sdk}_LIB_SUBDIR}-${SWIFTLIB_SINGLE_ARCHITECTURE}>)
+      target_sources(${target}
+                     PRIVATE
+                       "${SWIFTLIB_DIR}/${SWIFTLIB_SINGLE_SUBDIR}/swiftrt${CMAKE_C_OUTPUT_EXTENSION}")
+      set_source_files_properties("${SWIFTLIB_DIR}/${SWIFTLIB_SINGLE_SUBDIR}/swiftrt${CMAKE_C_OUTPUT_EXTENSION}"
+                                  PROPERTIES
+                                    GENERATED 1)
     endif()
   endif()
   _set_target_prefix_and_suffix("${target}" "${libkind}" "${SWIFTLIB_SINGLE_SDK}")