altos: Replace __critical usage with ao_arch_critical as needed

altos: Replace __critical usage with ao_arch_critical as needed

sdcc offers __critical as a machine-independent way to block
interrupts, but as gcc doesn't, we need to use a compiler-independent
construct instead. ao_arch_critical has been around since the AVR
port, but some old __critical usages remained.

This fixes a bunch of random hangs when communicating with MM over USB
or the radio as the various stdio loops were running without
interrupts blocked between the test and the sleep.

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