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_options.def b/runtime/runtime_options.def
index 1409a4e..146afc7 100644
--- a/runtime/runtime_options.def
+++ b/runtime/runtime_options.def
@@ -120,6 +120,7 @@
 RUNTIME_OPTIONS_KEY (ExperimentalFlags,   Experimental,     ExperimentalFlags::kNone) // -Xexperimental:{none, agents}
 RUNTIME_OPTIONS_KEY (std::vector<ti::Agent>,         AgentLib)  // -agentlib:<libname>=<options>, Requires -Xexperimental:agents
 RUNTIME_OPTIONS_KEY (std::vector<ti::Agent>,         AgentPath)  // -agentpath:<libname>=<options>, Requires -Xexperimental:agents
+RUNTIME_OPTIONS_KEY (std::vector<Plugin>,            Plugins)  // -Xplugin:<library> Requires -Xexperimental:runtime-plugins
 
 // Not parse-able from command line, but can be provided explicitly.
 // (Do not add anything here that is defined in ParsedOptions::MakeParser)