Kernel: Assert on page fault during IRQ

We're not equipped to deal with page faults during an IRQ handler,
so add an assertion so we can immediately tell what's wrong.

This is why profiling sometimes hangs the system -- walking the stack
of the profiled thread causes a page fault and things fall apart.

Kernel/Arch/i386/CPU.h

@@ -592,4 +592,6 @@ private:
     u32 m_flags;
 };
 
+extern u32 g_in_irq;
+
 }