Instead of blocking the triggering task when core 1 is still busy
handling the previous interrupt, offload interrupt dispatching into
a separate task.  This is the first step towards making API-calls
interrupt safe.

Next to the async triggering, the synchroneous mechanism is retained for
special cases where async does not work (e.g. because of spin-locks).
Currently, there is only one such case when resetting core 1 (triggering
EPIC_INT_RESET).

Signed-off-by: Rahix <rahix@rahix.de>