altos/stm: Allow use basepri instead of primask for masking interrupts

altos/stm: Allow use basepri instead of primask for masking 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.

Select this by defining AO_NONMASK_INTERRUPT in ao_pins.h.
non-maskable interrupt priority is AO_STM_NVIC_NONMASK_PRIORITY

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