Weak reference support

[qinsoon]

This adds weak reference support, and updates MMTk core to mmtk/mmtk-core#564.

• if weak reference is enabled in MMTk, we will add weak references during our tracing. Otherwise, still treat them as strong references.
• implement new changes in Weak reference processing mmtk-core#564
• add test with weak reference enabled (by default, it is disabled).

[wenyuzhao]

FYI this is the change that fixed the weak processor wenyuzhao@1bd175f. This is also a part of #141. This change removes the code that treats all the WeakProcessor objects as roots. I'm afraid without this fix, a lot of weak objects will be kept alive. We should include the change in this PR to deal with weak pointers correctly. Probably this will also yield better performance.

[qinsoon]

FYI this is the change that fixed the weak processor wenyuzhao@1bd175f. This is also a part of #141. This change removes the code that treats all the WeakProcessor objects as roots. I'm afraid without this fix, a lot of weak objects will be kept alive. We should include the change in this PR to deal with weak pointers correctly. Probably this will also yield better performance.

@wenyuzhao It seems, to apply the changes in the commit, I need to remove the use of MMTkRootScanWorkScope. Otherwise, I will see must not call oops_do_marking_prologue twice in a row.

[wks]

LGTM