Define per-target OSX_ARCHITECTURES property

The CMAKE_OSX_ARCHITECTURES variable works only as a global setting.
This commit defines target properties

  OSX_ARCHITECTURES
  OSX_ARCHITECTURES_<CONFIG>

to specify OS X architectures on a per-target and per-configuration
basis.  See issue #8725.

Source/cmTarget.h

@@ -410,6 +410,9 @@ public:
   // until we have per-target object file properties.
   void GetLanguages(std::set<cmStdString>& languages) const;
 
+  /** Get the list of OS X target architectures to be built.  */
+  void GetAppleArchs(const char* config, std::vector<std::string>& archVec);
+
   /** Return whether this target is an executable with symbol exports
       enabled.  */
   bool IsExecutableWithExports();