IRQ -> SVC32

[Matt Thomas <matt%3am-software.com@localhost>]

When an ARM get an IRQ (interrupt), it starts running in IRQ mode but 
immediately
switches to SVC32 mode.

I was thinking about this and the only reason I can see to switch to SVC32
is when an AST needs to serviced.  But other than that, we should be able
to do everything in IRQ mode.

This made cpu_intr_p() even faster since we just test the current mode
is FIQ or IRQ and we don't need to fetch memory.