Fixed race condition in setting dd_new_allocation

Fixed race condition in setting dd_new_allocation

Sequence:

+ At the end of BGC we call compute_new_dynamic_data then switch to
preempt, at this point dd_new_allocation isn't set yet;
+ an FGC happens and saves new_allocation in gc_new_allocation
+ FGC finishes and BGC keeps running and saves gc_new_allocation in
new_allocation

The fix is to finish filling in the values for new_allocations before
we allow an FGC to happen.

Should really get rid of gc_new_allocation in the future. I am making
this fix to unblock my work on the fragmentation stuff.

Commit migrated from https://github.com/dotnet/coreclr/commit/43c102efb157c6b6c39457284a17473388822725