VS: Use internal abstraction for VCTargetsPath host arch

Call our internal host architecture lookup method rather than directly
accessing a member used by its implementation.

Source/cmGlobalVisualStudio10Generator.cxx

@@ -786,10 +786,9 @@ bool cmGlobalVisualStudio10Generator::FindVCTargetsPath(cmMakefile* mf)
     }
     cmXMLElement(eprj, "Import")
       .Attribute("Project", "$(VCTargetsPath)\\Microsoft.Cpp.Default.props");
-    if (!this->GeneratorToolsetHostArchitecture.empty()) {
+    if (const char* hostArch = this->GetPlatformToolsetHostArchitecture()) {
       cmXMLElement epg(eprj, "PropertyGroup");
-      cmXMLElement(epg, "PreferredToolArchitecture")
-        .Content(this->GeneratorToolsetHostArchitecture);
+      cmXMLElement(epg, "PreferredToolArchitecture").Content(hostArch);
     }
     {
       cmXMLElement epg(eprj, "PropertyGroup");