[POWERPC] Remove #ifdef around set_dabr in signal code

set_dabr() and thread.dabr exist on 32 bits as well nowadays (they
actually may do something even, depending on what CPU you have).

So this removes the ifdef.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>