Use do statement in PDBG and PWRN

The do statement ensures the macros to generate just one expression that
is compatible with any programming construct. The concrete bug was

  if (cond)
    PDBG(...);
  else
    ...

which was expanded to

  if (cond)
    if (DO_PDBG)
      Genode::printf(...);
  else
    ...

This is obviously wrong as the *else* branch is then connected to the
second *if*.