Add basic runtime-plugins support.

This allows one to pass shared-libraries on the command line that the
runtime will load as plugins. They have access to runtime code and can
install hooks to add functionality. Currently the only hook they can
touch is JavaVMExt::AddEnvironmentHook to register a callback for
GetEnv(). More hooks might be added in the future.

Test: ./test/run-test 900
Change-Id: I852b4daf5a3fa71e9888722bc07794632c0e5010
diff --git a/runtime/runtime.h b/runtime/runtime.h
index 10cc960..6da60f2 100644
--- a/runtime/runtime.h
+++ b/runtime/runtime.h
@@ -83,6 +83,7 @@
 class MonitorPool;
 class NullPointerHandler;
 class OatFileManager;
+class Plugin;
 struct RuntimeArgumentMap;
 class SignalCatcher;
 class StackOverflowHandler;
@@ -702,6 +703,7 @@
   std::vector<std::string> properties_;
 
   std::vector<ti::Agent> agents_;
+  std::vector<Plugin> plugins_;
 
   // The default stack size for managed threads created by the runtime.
   size_t default_stack_size_;