Update the MMTk core depencency

We update MMTk core to the latest revision.

The MMTk core removed the `is_mmtk_object` Cargo feature, but the
function that supports conservative stack scanning is still exposed via
the `vo_bit` feature.

The MMTk core also made a change that allows the side metadata memory to
be dynamically determined by `mmap` during start-up.  This doesn't
affect the mmtk GC module, yet, because we are not accessing any side
metadata (particularly VO bits) directly.

Author: wks