[Qemu-ppc] Missing POWER9 addex

[Torbjörn Granlund]

The addex instruction seems to be missing from qemu, even from the 3.1
snapshots.

We use this instruction in the latest GMP snapshots, leading to illop
signals.

This is an instruction added to P9.  It is akin to adde but uses the OV
bit for carry in/out instead of the CA bit.


Please note that there is a slight oddity with this instruction's
coding.  The secondary opcode is 170, but it is documented as coded in
bits 23 through 30.  Other arithmetic insns' secondary opcode is placed
in bits 22 through 30.

It might be neater to treat addex like other arithmetic insns, and put
the opcode 170 in bits 22..30.  That would make bit 22 zero, but since
the CY field in bits 21..22 needs to be 00 currently (other values are
reserved) this should not matter.


-- 
Torbjörn
Please encrypt, key id 0xC8601622