review for 7029167: add support for conditional card marks

[Tom Rodriguez]

http://cr.openjdk.java.net/~never/7029167

7029167: add support for conditional card marks
Reviewed-by:

Repeated updates to the card table can be a source of contention on
multiprocessor machines that can greatly inhibit scalability.  This
adds support in C2 for conditional card marks.  Tested with jbb and
CTW.  At some point this should be turned on automatically when multi
socket machines are detected but that's covered under another bug as
is C1 support.