target/mips: add Octeon V3MULU instruction

V3MULU extends VMULU across the full Octeon3 multiplier state, adding rt
and queued partial products.

Return the low result while shifting the remaining accumulated limbs back
into P[0] through P[5].

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20260520172313.23777-23-philmd@linaro.org>