Backport PR#127 PR#144. Update mmtk-core. (#145)

This backports #127 and #144 to v1.9.2+RAI, and updates mmtk-core.

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Co-authored-by: Kunshan Wang <wks1986@gmail.com>
Co-authored-by: mmtkgc-bot <mmtkgc.bot@gmail.com>

Author: 3 people