target/mips: add Octeon SAAD instruction

SAAD is the doubleword form of SAA: it atomically adds rt to the
naturally aligned 64-bit doubleword at base and discards the old memory
value.

Route it through the common SAA/SAAD translator so the MemOp selects the
aligned doubleword transaction size.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20260520172313.23777-15-philmd@linaro.org>