ntdll: Only declare/set wow_teb on platforms using it
Otherwise we see dlls/ntdll/unix/signal_x86_64.c:2351:14: warning: unused variable 'wow_teb' on FreeBSD.
Otherwise we see dlls/ntdll/unix/signal_x86_64.c:2351:14: warning: unused variable 'wow_teb' on FreeBSD.