util/bufferiszero: Use i386 host/cpuinfo.h

Use cpuinfo_init() during init_accel(), and the variable cpuinfo
during test_buffer_is_zero_next_accel().  Adjust the logic that
cycles through the set of accelerators for testing.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>