trace: reimplement the DTRACE function so it doesn't use a vector

At the same time, rename the trace flags to debug flags since they
have broader usage than simply tracing.  This means that
--trace-flags is now --debug-flags and --trace-help is now --debug-help

src/cpu/thread_context.cc

@@ -32,6 +32,7 @@
 #include "base/trace.hh"
 #include "config/the_isa.hh"
 #include "cpu/thread_context.hh"
+#include "debug/Context.hh"
 
 void
 ThreadContext::compare(ThreadContext *one, ThreadContext *two)