RFR: 8197454: Need Access decorator for storing oop into uninitialized location

[Kim Barrett]

Please review this change to the Access API to support stores of oops
into uninitialized locations.  This change is needed to prevent such
stores from, for example, having the G1 pre-barrier applied to
whatever garbage happens to be in the location being stored into.

There was already support for stores to uninitialized locations in the
Access API, but only for array initialization.  This change
generalizes that mechanism, and renames it accordingly:
ARRAYCOPY_DEST_NOT_INITIALIZED => AS_DEST_NOT_INITIALIZED.

CR:
https://bugs.openjdk.java.net/browse/JDK-8197454

Webrev:
http://cr.openjdk.java.net/~kbarrett/8197454/open.00/

Testing:
Mach5 {hs,jdk}-tier{1,2,3}