Teach the call_time trace to notice when the trace dies (non-SMP system)

The call_time trace is a special kind of tracing that requires
a tracer process just like ordinary call trace, but it never
actually sends anything to the tracer. It merely use the existence
of a trace process (and call trace flags) as an indication that
call_time tracing is active for the process.

If the tracer dies in a non-SMP run-time system, processes with
call_time tracing would not notice that the tracer had
died. Furthermore, if the set_on_spawn flag was active, the dead
tracer could be propagaged to newly spawned processes.

Before accumulating trace information in a non-SMP system, always
validate the tracer process. (In an SMP system, a reference to a
dead tracer will be cleared away each time a process is scheduled.)

While we could put all of the new code beam_bp.c, we have chosen to
make a function call from beam_bp.c to a function in erl_trace.c for
clarity's sake and to ease further maintenance. In the future, we
might want to handle tracing in more similar ways in the SMP and
non-SMP system.

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