[drlvm][gc]GC options update

[Chen Yang]

Hi All:
   We have updated GC options in recent check-in:
   For minor algorithm, option can be: -XX:
gc.minor_algorithm=partial_forward or semi_space
   For major algorithm, option can be:
-XX:gc.major_algorithm=slide_compactor move_compact or mark_sweep
   The current default is partial_forward and move_compact for minor and
major. You don't need to specify anything
if you don't know what I am talking. :-)
   Thanks.
--
 Chen