Adding --target= and --trace_registers flags.

--target=[xex|iso] makes it possible to specify the thing to run from a
flagfile.

src/xenia/cpu/x64/x64_emitter.cc

@@ -228,7 +228,9 @@ int X64Emitter::MakeFunction(FunctionSymbol* symbol) {
   int result_code = 1;
   Lock();
 
-  XELOGCPU("Compile(%s): beginning compilation...", symbol->name());
+  if (FLAGS_log_codegen) {
+    XELOGCPU("Compile(%s): beginning compilation...", symbol->name());
+  }
 
   symbol_ = symbol;
   fn_block_ = NULL;