Add the signal catcher (for SIGQUIT and SIGUSR1).

Also fix some of the thread implementation.

Change-Id: If2d1b59a149ba1ac192ad9bc74319c8dff228549
diff --git a/src/class_linker.cc b/src/class_linker.cc
index 04a17cf..3907d07 100644
--- a/src/class_linker.cc
+++ b/src/class_linker.cc
@@ -1920,4 +1920,9 @@
   return resolved;
 }
 
+size_t ClassLinker::NumLoadedClasses() const {
+  MutexLock mu(classes_lock_);
+  return classes_.size();
+}
+
 }  // namespace art