target/riscv: support for 128-bit arithmetic instructions

target/riscv: support for 128-bit arithmetic instructions

Addition of 128-bit adds and subs in their various sizes,
"set if less than"s and branches.
Refactored the code to have a comparison function used for both stls and
branches.

Signed-off-by: Frédéric Pétrot <frederic.petrot@univ-grenoble-alpes.fr>
Co-authored-by: Fabien Portas <fabien.portas@grenoble-inp.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20220106210108.138226-14-frederic.petrot@univ-grenoble-alpes.fr
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>