CMake: move the Platform (Darwin, Glibc) library under the SDK overlay condition

Author: gribozavr

stdlib/public/CMakeLists.txt

@@ -31,10 +31,13 @@ if(SWIFT_BUILD_STDLIB)
   add_subdirectory(stubs)
   add_subdirectory(core)
   add_subdirectory(SwiftOnoneSupport)
-  add_subdirectory(Platform)
   add_subdirectory(Reflection)
 endif()
 
+if(SWIFT_BUILD_SDK_OVERLAY)
+  add_subdirectory(Platform)
+endif()
+
 if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
   if(SWIFT_BUILD_SDK_OVERLAY)
     add_subdirectory(SDK)