[POWERPC] Consolidate restore_sigmask

restore_sigmask is exactly the same on 32 and 64bit, so move it to
common code.  Also move _BLOCKABLE to signal.h to avoid defining it
multiple times.

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>