RFR(S) 8249276 CDS archived objects must have "neutral" markwords

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8249276
http://cr.openjdk.java.net/~iklam/jdk16/8249276-reset-archive-obj-headers.v01/

Please review this change (initial patch provided by David Holmes; I 
rearranged it a little and added more comments).

We reinitialize the markWord of all archived objects to remove any side 
effect of GC/locking/etc during "java -Xshare:dump".

(David mentioned in the bug comments about assertions inside the 
identity_hash() call, but maybe this should be fixed in a different bug?)

Tested with mach5 tiers 1-4.

Thanks
- Ioi