ppc: Fix signal delivery in ppc-user and ppc64-user

ppc: Fix signal delivery in ppc-user and ppc64-user

There were a number of bugs in the implementation:

 - The structure alignment was wrong for 64-bit.

 - Also 64-bit only does RT signals.

 - On 64-bit, we need to put a pointer to the (aligned) vector registers
   in the frame and use it for restoring

 - We had endian bugs when saving/restoring vector registers

 - My recent fixes for exception NIP broke sigreturn in user mode
   causing us to resume one instruction too far.

 - Add VSR second halves

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>