[POWERPC] Consolidate do_signal

do_signal has exactly the same behaviour on 32bit and 64bit and 32bit
compat on 64bit for handling 32bit signals.  Consolidate all these
into one common function in signal.c.  The only odd left over is
the try_to_free in the 32bit version that no other architecture has
in mainline (only in i386 for some odd SuSE release).  We should
probably get rid of it in a separate patch.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>