Make ModifyCardsAtomic 64 bit safe.

There was some logic which assumed sizeof(uintptr_t) == 4 bytes.
New method also provides a slight speedup in
FormulaEvaluationActions.EvaluateAndApplyChanges

AllocSpaceClearCards:

Before total time spent aging alloc space cards:
914ms @ 389 GC iterations
886ms @ 386 GC iterations
906ms @ 413 GC iterations

After:
865ms @ 376 GC iterations
857ms @ 386 GC iterations
826ms @ 379 GC iterations

Change-Id: Ie375b7e57c36a0257a4ffe5a1622ffb6754509c7