altos: Avoid modulus in task queue on parts without idiv

altos: Avoid modulus in task queue on parts without idiv

Cortex-M0 parts don't have a hardware divide, so doing a modulus
to compute task hash indexes is super slow. Avoid that by making the sleep
queue a power of two.

Signed-off-by: Keith Packard <keithp@keithp.com>