Improve debug-ability of native method registration

If we fail to register a native we construct a stack trace which needs
Throwable and stack trace support. Registering Throwable's natives first
and making Throwable a root supports this. Previously we could only
generate a stack trace when thread start up was complete.

Change-Id: Icc7c1657988c7f36aceb1d4f3e61e7218f55e055
diff --git a/src/class_linker.h b/src/class_linker.h
index 10d8305..f2b44a3 100644
--- a/src/class_linker.h
+++ b/src/class_linker.h
@@ -429,6 +429,7 @@
     kJavaLangClassLoader,
     kDalvikSystemBaseDexClassLoader,
     kDalvikSystemPathClassLoader,
+    kJavaLangThrowable,
     kJavaLangStackTraceElement,
     kPrimitiveBoolean,
     kPrimitiveByte,