base: remove Trace::enabled flag

The DTRACE() macro tests both Trace::enabled and the specific flag. This
change uses the same administrative interface for enabling/disabling
tracing, but masks the SimpleFlags settings directly. This eliminates a
load for every DTRACE() test, e.g. DPRINTF.

util/cxx_config/main.cc

@@ -117,7 +117,7 @@ main(int argc, char **argv)
     Stats::initSimStats();
     Stats::registerHandlers(CxxConfig::statsReset, CxxConfig::statsDump);
 
-    Trace::enabled = true;
+    Trace::enable();
     setDebugFlag("Terminal");
     // setDebugFlag("CxxConfig");