Windows: Add GetActiveProcessorCount stub

It's required by recent libc++.

Source/Windows/Common/WinAPI/Misc.cpp

@@ -184,3 +184,7 @@ DLLEXPORT_FUNC(LONG, RegGetValueA, (HKEY hKey, LPCSTR lpSubKey, LPCSTR lpValue,
 DLLEXPORT_FUNC(LONG, RegCloseKey, (HKEY hKey)) {
   return RtlNtStatusToDosError(NtClose(hKey));
 }
+
+DLLEXPORT_FUNC(DWORD, GetActiveProcessorCount, (WORD group)) {
+  UNIMPLEMENTED();
+}