[sgen] Fix cardtable for huge (>4gb) objects on 64bit.

We use an overlapping card table on 64 bits, meaning that the address
space wraps around our card table (every 4gb).  We had assumed in a few
places that an object could wrap around the end of the card table at most
once, but with objects larger than 4gb that is not the case.

Fixes the corlib test suite crash on OSX/64bit
(in SortedListTest.TestCapacity4).