Tracing in interrupt service routines can easily deadlock the kernel
(c.f. c4183db). Let's be less dumb
about it.

This commit does the following:

- Factor out the code for forcefully unlocking the mutices around
  tracing outputs (VGA and serial) into a separate, unsafe function, and
  make the ISRs that are (currently) always a kernel oops just call that
- Remove the trace in the "spurious interrupt" handler, as that seems
  like a good way to deadlock the universe. In future, let's have a
  way to track whether spurious interrupts have fired that doesn't
  involve emitting a trace (maybe counters?)