Currently armv7m_nvic_acknowledge_irq() does three things:
* make the current highest priority pending interrupt active
* return a bool indicating whether that interrupt is targeting
Secure or NonSecure state
* implicitly tell the caller which is the highest priority
pending interrupt by setting env->v7m.exception
We need to split these jobs, because v7m_exception_taken()
needs to know whether the pending interrupt targets Secure so
it can choose to stack callee-saves registers or not, but it
must not make the interrupt active until after it has done
that stacking, in case the stacking causes a derived exception.
Similarly, it needs to know the number of the pending interrupt
so it can read the correct vector table entry before the
interrupt is made active, because vector table reads might
also cause a derived exception.
Create a new armv7m_nvic_get_pending_irq_info() function which simply
returns information about the highest priority pending interrupt, and
use it to rearrange the v7m_exception_taken() code so we don't
acknowledge the exception until we've done all the things which could
possibly cause a derived exception.
Backports part of commit 6c9485188170e11ad31ce477c8ce200b8e8ce59d from qemu
e1349f817a