Add visiting for class loaders in StickyMarkSweep

StickyMarkSweep clear the mark stack,
Because all reachable objects must be referenced by a root or a dirty card.
But, there are some marking hole for class object.

If some object is marked and the object and it's class object is not dirty,
Then class object cannot be marking by card table.

In previous OS including mashmellow,
Class table was maintaned by class linker
and all class object was marked with kVisitRootFlagAllRoots flag.

In N OS,
Class object is not marked with kVisitRootFlagAllRoots.
So, I added new flag to mark class object and using it StickyMarkSweep.

Added regression test in 141-class-unload.

Test: test-art-host
Merged-In: I57599e6db53b260f4c5ef466b63962141b8da5c3

(cherry picked from commit a2d1b285)

Bug: 33924225

Change-Id: I57599e6db53b260f4c5ef466b63962141b8da5c3
Signed-off-by: Hyangseok Chae <neo.chae@lge.com>