The goal is to avoid a situation where core 1 is shut down while holding
the tinyusb spinlock, which could happen during soft reset if
mp_thread_deinit is called while core1 is running tud_task().
This also fixes a latent race where the two cores are competing to
decrement and compare `vm_hook_divisor` with no mem fence or atomic
protection -- only core0 will now do this.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>
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