Implement the addex instruction introduced in ISA V3.00 in qemu tcg.

The add extended using alternate carry bit (addex) instruction performs
the same operation as the add extended (adde) instruction, but using the
overflow (ov) field in the fixed point exception register (xer) as the
carry in and out instead of the carry (ca) field.

The instruction has a Z23-form, not an XO form, as follows:

    ------------------------------------------------------------------
    |   31   |   RT   |   RA   |   RB   |   CY   |     170     |  0  |
    ------------------------------------------------------------------
    0        6        11       16       21       23            31    32

However since the only valid form of the instruction defined so far is
CY = 0, we can treat this like an XO form instruction.

There is no dot form (addex.) of the instruction and the summary overflow
(so) bit in the xer is not modified by this instruction.

For simplicity we reuse the gen_op_arith_add function and add a function
argument to specify where the carry in input should come from and the
carry out output be stored (note must be the same location).

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>