VS: Delay getting platform name in local generator

Ask the global generator during generation instead of trying
to store it up front.  Later the global generator may not know
the platform name when it is creating the local generator.

Source/cmLocalVisualStudio7Generator.h

@@ -53,8 +53,6 @@ public:
    */
   void SetBuildType(BuildType,const std::string& name);
 
-  void SetPlatformName(const std::string& n) { this->PlatformName = n;}
-
   void SetExtraFlagTable(cmVS7FlagTable const* table)
     { this->ExtraFlagTable = table; }
   virtual std::string GetTargetDirectory(cmTarget const&) const;
@@ -124,7 +122,6 @@ private:
   std::string ModuleDefinitionFile;
   bool FortranProject;
   bool WindowsCEProject;
-  std::string PlatformName; // Win32 or x64
   cmLocalVisualStudio7GeneratorInternals* Internal;
 };