Disable adding main and non moving spaces to immune region in GSS

Disabled adding the main and non moving space to the immune region.
This will enable us to recycle bump pointer spaces for malloc space
-> malloc space compaction as well as collector transitions.

Also added logic for falling back to the non moving space, we may
copy objects there.

Refactored mod union table logic into MarkReachableObjects.

No measurable performance benefit or regression.

Bug: 14059466
Bug: 16291259

Change-Id: If663d9fdbde943b988173b7f6ac844e5f78a0327
diff --git a/runtime/class_linker.h b/runtime/class_linker.h
index d9b3d25..d6cceff 100644
--- a/runtime/class_linker.h
+++ b/runtime/class_linker.h
@@ -634,7 +634,7 @@
   // retire a class, the version of the class in the table is returned and this may differ from
   // the class passed in.
   mirror::Class* EnsureResolved(Thread* self, const char* descriptor, mirror::Class* klass)
-      __attribute__((warn_unused_result)) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+      WARN_UNUSED SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
   void FixupTemporaryDeclaringClass(mirror::Class* temp_class, mirror::Class* new_class)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);