Call through powerpc_interrupt for all Book-E interrupts

Make int_external_input, int_decrementer, and int_performance_counter all
now use trap_common, just like on AIM. The effects of this are:

• All traps are now properly displayed in ddb. Previously traps from external input, decrementer, and performance counters, would display as just basic stack traces. Now the frame is displayed.

• External interrupts are now handled with interrupts enabled, so handling can be preempted. This seems to fix a hang found post-r329882.