Integrated Chrome tracing

Added support for chrome tracing by integrating the
minitrace library (github.com/hrydgard/minitrace),
and modified it with a background flushing thread that allows
tracing to continue automatically after the predefined
1 million event limit.

Menu items and an accelerator (Ctr+T) have also been
added. Starting and stopping the trace simply replaces
trace.json with the new trace data.

The feature is disabled by default. Pass MINITRACE=y
to the build command to enable it. Some traces are
already added as an example, however they won't have
any effect if the feature is disabled.

src/pc.c

@@ -89,6 +89,9 @@ int		dopause,			/* system is paused */
 uint64_t	timer_freq;
 char		emu_version[200];		/* version ID string */
 
+#ifdef MTR_ENABLED
+int     tracing_on = 0;
+#endif
 
 /* Commandline options. */
 int	dump_on_exit = 0;			/* (O) dump regs on exit */