Tweak struct avr_t to always include a pointer to trace data structure.

Tweak struct avr_t to always include a pointer to trace data structure.

Previously the size of struct avr_t was dependent on whether the
preprocessor macro CONFIG_SIMAVR_TRACE was defined. This was problematic
since it meant that applications that link to simavr need to define it
the same way as it is defined in simavr Makefiles.

Change it to a single pointer to a struct, which is allocated iff
tracing is compiled in and a NULL pointer otherwise.

Signed-off-by: Sami Liedes <sliedes@cc.hut.fi>