Make object forwarding VM specific

[qinsoon]

Currently the forwarding code is in mmtk-core. We figure out where we can store the forwarding pointer from the GC_BYTE_OFFSET. For #276, we may need to remove GC_BYTE_OFFSET. That means we may need to make object forwarding VM-specific.

[qinsoon]

Related issues/PRs:

• Let the VM customize object forwarding implementation #975
• Forwarding pointer is not an object metadata #1030

[steveblackburn]

Closing because it is covered by #1030.

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