Immix GC straddling object optimisation

[wenyuzhao]

Add OpenJDK fast path code to log straddling objects. Do this after Immix PR is merged.

[qinsoon]

In Java MMTk, if an object is straddling lines, we set a bit for the object to know that it is a straddling object. And when we mark lines, we do this:

mark_one_line
if straddling
  mark_multiple_lines

In our current code, we always do mark_multiple_lines which always calls get_current_size() for the object.