altos/stm: Use basepri instead of primask for controlling interrupts

altos/stm: Use basepri instead of primask for controlling interrupts

This allows for high priority interrupts (priority 0) to run, even
when other interrupts are blocked. Code executing in such interrupt
handlers must not attempt to control task execution as that will race
with the scheduler.

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