Create a basic agent loading framework.

Currently we only allow agents to be loaded at runtime startup, though
this is expected to change soon.

Test: ./test/run-test --host 900
Change-Id: Id648eaed4bbbe6fdef41d64922d023a4db0bfa54
diff --git a/runtime/base/logging.h b/runtime/base/logging.h
index 6323eee..ac21a3f 100644
--- a/runtime/base/logging.h
+++ b/runtime/base/logging.h
@@ -57,6 +57,7 @@
   bool verifier;
   bool image;
   bool systrace_lock_logging;  // Enabled with "-verbose:sys-locks".
+  bool agents;
 };
 
 // Global log verbosity setting, initialized by InitLogging.