When the pipeline is enabled, the co-kernel may switch contexts over
the irq stack, and more interrupts can be taken over sibling stack
contexts.

Therefore, we cannot depend on the current stack pointer for figuring
out:

- the value of the "current task" register (sp_el0) in the context
  switching code, since %sp may live in the irq stack.

- whether the irq stack was previously exited when taking a new
  interrupt, since a context switch may have occurred earlier over the
  irq stack to a co-kernel task.