[POWERPC] Consolidate sys_sigaltstack

sys_sigaltstack is the same on 32bit and 64 and we can consolidate it
to signal.c.  The only difference is that the 32bit code uses ints
for the unused register paramaters and 64bit unsigned long.  I've
changed it to unsigned long because it's the same width on 32bit.

(I also wonder who came up with this awkward calling convention.. :))

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>