Previously this was not set, so potential for race conditions in interrupt
handlers this didn't issue SEV. (Which is currently all of them, as far as
I can see.)
Eventually we might be able to augment the interrupt handlers that wake the
main thread to call SEV, and leave the others as-is to suspend the CPU
slightly faster, but this will solve the issue for now.
This work was funded through GitHub Sponsors.
Signed-off-by: Angus Gratton <angus@redyak.com.au>
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