EABI unwinder enhancements... When it's time to stop unwinding, don't
exit the loop until after printing info about the current frame.  Also,
if executing the unwind function for a frame doesn't change the values of
any registers, log that and exit the loop rather than looping endlessly.
Insert STOP_UNWINDING directives in the _start (kernel entry point) and
fork_trampoline (thread entry point) assembler routines, because it's
not possible to unwind beyond those points.

Also insert STOP_UNWINDING in the exception_exit routine, to prevent an
unwind-loop at that point.  This is just a stopgap until we get around
to instrumenting all assembler functions with proper unwind metadata.
Fix comment block formatting.