altos: Add ao_task_minimize_latency to reduce IRQ delays

When set, this causes the task switching code to avoid blocking IRQs
while looking for an idle task as that can increase IRQ latencies
enough to drop characters at 115200 baud on the cc1111. Note that this
*also* eliminates the ability to use low power modes as we cannot know
at any point whether some interrupt has come along and woken a task.

Has no effect when using task queues as those require IRQs to be
blocked while looking at the queue. Shouldn't be a problem there
though as the check for no running tasks is very cheap.

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