in the window between the beginning of panic() and entering the debugger,
it's possible to receive interrupts. If we receive an interrupt, don't
preempt if panicstr != NULL, as the system is in the process of failing, and
the preempting thread is likely to stumble over the failure. The typical
scenario is during the printf() in panic() prior to entering the debugger,
but when running with a slower console type such as serial console.
It could be that the panic string should be passed to the debugger to print,
so that it can run from the debugger's environment rather than a regular
kernel printf.
Glanced at by: jhb
bc60830675