Add simple function-tracing support. Prints a message

Add simple function-tracing support.  Prints a message
every time the committed PC changes from one symbol scope
to another.
Set function_trace=y on target CPU to enable.
To defer start, use function_trace_start=<tick>
(in addition to setting function_trace=y).

cpu/base_cpu.cc:
cpu/base_cpu.hh:
    Add simple function-tracing support.
cpu/simple_cpu/simple_cpu.cc:
    Add function_trace, function_trace_start params
    Call traceFunctions() on instruction completion
cpu/simple_cpu/simple_cpu.hh:
    Add function_trace, function_trace_start params

--HG--
extra : convert_revision : 8a7f84028ccbaee585253629007f32fc8eae35e1