fault: pass power led object to led functions

In the fault handlers, pass the power led object into the led_toggle()
calls, rather than NULL.  This fixes hard faults occuring on standard
faults or panics, due to a NULL dereference.

For ARMv7-M put standard fault behavior in begin_fatal_fault() and
end_fatal_fault(), called by all of the fault handlers.

Change-Id: I855b7b66a7d7881aba6efc7c2dc9a1ade46c2048