It is UB to pass `entry` to core1 as `&mut`, because core0 keeps
an aliasing pointer to that memory region, and actually writes to
it (when `spawn_core1` returns, the stack frame gets deallocated and the memory
gets reused). This violates noalias requirements.
Added the fence just in case, een though it works without.
doing this setup work repeatedly, on every wait, is unnecessary. with
nothing ever disabling the interrupt it is sufficient to enable it once
during device init and never touch it again.
