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.

src/python/m5/trace.py

@@ -32,7 +32,7 @@ import util
 from internal.trace import output, ignore
 
 def disable():
-    internal.trace.cvar.enabled = False
+    internal.trace.disable()
 
 def enable():
-    internal.trace.cvar.enabled = True
+    internal.trace.enable()