Handle IRQs in system mode.

This IRQ routine switches into system mode so that ISRs can share the
same banked registers with user programs. This is especially important
for ctxsw(), wich needs to manipulate SP_usr.

I'm now able to run the clockTest(), amongst a few others, rapidly
without crashing.

Unfortunately, it looks like the timesliceThreadTest() no longer works
with this change. I'll have to resolve that before merging this branch
in.