[POWERPC] Less ifdef's in signal.c/signal.h

This patch moves things around a little bit in the new common signal.c
and signal.h files to remove the last #ifdef in the middle of the
common do_signal().

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