arch-power: Add doubleword multiply-add instructions

This introduces 128-bit addition helpers and adds the
following instructions.
  * Multiply-Add Low Doubleword (maddld)
  * Multiply-Add High Doubleword (maddhd)
  * Multiply-Add High Doubleword Unsigned (maddhdu)

Change-Id: I04e6ea5fb4978b341a6e648424de2930ad41f449
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40905
Reviewed-by: Boris Shingarov <shingarov@labware.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
4 files changed