ART: Add OnLoad system classloader search support

Add support for extension of the system classloader search path
during the OnLoad phase. Add test.

Bug: 34359699
Test: m test-art-host-run-test-936-search-onload
Change-Id: Ie0a5b4bb77999dfae87f955dde40b73ffe053b8e
diff --git a/runtime/openjdkjvmti/ti_phase.h b/runtime/openjdkjvmti/ti_phase.h
index bd15fa6..851fc27 100644
--- a/runtime/openjdkjvmti/ti_phase.h
+++ b/runtime/openjdkjvmti/ti_phase.h
@@ -57,6 +57,8 @@
 
   struct PhaseCallback;
 
+  static jvmtiPhase GetPhaseUnchecked();
+
  private:
   static jvmtiPhase current_phase_;
 };