Add x64 configuration to Windows VC90 project files

 * Add the x64 Platform for all Configuration types
 * This adjusts the output and intermediate directories for all builds to be
   "$(PlatformName)\$(ConfigurationName)", so Win32 DebugDLL builds go in
   'builds\msvc\Win32\DebugDLL' and x64 ReleaseDLL builds go in
   'builds\msvc\x64\Release'.  This allows all variants to be built without
   stomping on each other.