util/bufferiszero: Simplify test_buffer_is_zero_next_accel

Because the three alternatives are monotonic, we don't need
to keep a couple of bitmasks, just identify the strongest
alternative at startup.

Generalize test_buffer_is_zero_next_accel and init_accel
by always defining an accel_table array.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>